This post mind sound a bit less fluffy/more heartless than others, but here it goes.
I worked as a technician at a shelter for about 2 years before I started doing research. I worked full time basically so I knew the ins and outs, every single animal (about 400 total) by name and face, did intakes/adoptions/vet runs/etc...so you could say I knew the place pretty well, haha.
I'm not sure how you describe this in words, but you need to find a way to be attached and not attached at the same time. If you don't "care" about the animals, you'll be miserable, you won't do as good a job taking care of them, and customers will think you're rude or stupid if you don't know anything about the animals because you spend as little time as possible with them.
If you get too attached though, you run the obvious risks of being sad if they die/get sick/get adopted, spend TOO MUCH time with them and lose track of your daily tasks, and you start to get incredibly picky on who gets to adopt "your favorite" animals, when, when it comes down to it, it's about getting the animals a decent home...even if it's not the "perfect" home.
With that said, you will have favorites. In the two years I worked there I had 2 doggie and 1 cat favorites: A 4 year old Rottie named Emilio, an 11 year old chow/newfie mix named Oso, and a 14 year old cat named Monty. Monty is still at the shelter last time I checked, he must be close to 15 now...it's a no kill shelter (which I have mixed feelings on), so animals sit there until they get adopted or die. It's depressing. It's sad. Get used to it. I don't mean that in a harsh way, just that if you don't accept the policies or conditions (assuming they are good conditions) of the shelter, you will hate it. When it came to my favorites I blatantly tried harder to adopt them to people I thought were good families/owners, and in the end the two dogs ended up in great homes (a woman who rescues rotties and one of our volunteers took Oso home).
The shelter I worked at was over capacity and under staffed. This means dogs only got out twice a day usually and cats were only taken out to have their kennel cleans - and some of the technicians who were scared of the more feral cats would skip taking them out and only clean around them and change their litter box, which was even more annoying. We only had volunteers on the weekends, maybe one or two people during the weekday so it was all on the 5 or 6 of us working to feed all 400 animals, let out all the dogs, spray down all the kennels, clean all the cat cages/rooms, do laundry, do dishes, help people wanting to turn in animal (someone came every other hour or so), and other menial tasks. Sounds like a lot? It is. Most things were done completely at the end of the day or were not done to the best of standard. I stayed late generally (did not get paid overtime) because I had a hard time leaving without all the animals being clean, but eventually my boss started getting pissy and made us leave right at 5pm regardless of what was going on. Did I mention we left at 5pm and didn't get in until 8am? That's 15 hours for the dogs to "hold it." A surprising number of them could hold it, others could not. The saving part of this was the dogs had very large runs divided into two seconds, one of which they would mess on while the side with their food/bedding/water stayed pretty clean except for puppy kennels and such obviously.
You're going to want to kill customers. Get used to it. People will come in with the dumbest criteria. "I don't like black cats, they scare me" "I only want this specific breed," etc. Most people come in for puppies or kittens and leave if you don't have any on the floor. Most people will ignore dogs over 6 years old or so, which is a shame because older dogs have always been the best behaved in my opinion. Most will reject a cat that scratches or bats at them even the tiniest bit because "it's mean" or "it doesn't want me to hold it, I don't want it." People will want to know every detail of a dog, including strays that we've only handled for 2 weeks and will get frustrated if you don't know how housebroken it is, does it like this, does it like that, etc. People will breed stereotype and walk right by pitbulls, German shepherds, and Rotties.
People will turn in animals for equally stupid reasons. A lot of people turned in animals because they were moving and couldn't keep them...when the recession hit we got a LOT of dogs people couldn't afford to feed. That is sad, but understandable I suppose. But lots of people would just drop off boxes overnight when we said we were full and couldn't take them, leaving us with the extra animals. Some would have unwanted litters and when I asked if the cat/dog was spayed now they'd say "Well, no, it's an inside animal that just got out once" Expect to see those people come back every breeding season. A lot of the older cats we got because "we didn't expect them to live that long."
You will deal with well behaved, aggressive, scared, shy, abused, injured, old/young/sick animals. Be prepared for people turning in animals to lie about health issues that creep up on you later. Be prepared for that dog that you think looks aggressive and the owner assures you "he's just hyper" to be actually aggressive later, which is why they turned them in.
Be prepared to put animals down, even if it's a no kill. My first day on the job we had to put Buddy, a big old chocolate lab down...actually he died on the way to the emergency clinic, pulmonary embolism. You will lose kittens in a weak litter, not all puppies will survive, neither will those sick looking strays someone leaves in a box. Cold winters? Be prepared for the animals left on your doorstep to be dead by the time you get into work in the morning.
A lot of this you get used to with time. The first few months I had a hard time doing intakes because people would cry or be very rude and get under my skin. After two years, nothing phased me. You could make up the worst sob story to give up your pet, cry at my feet...nothing. It's not that I don't sympathize, for a lot of them I did. But it stopped getting under my skin. The first time a dog came in shot or hurt and left for dead it ruined my day. By the end of two years, it's just usual business.
If you're still feeling down about all this in a couple months, my honest advice is quit. Or go from working to volunteering...I volunteered for a few years before I became an employee, and I had loads of fun. I could clean/visit/socialize with the animals on my own schedule. It wasn't until I became staff that I realized all the drama/heartbreak/sweat and blood that comes with working at a place like that. I'm not saying you're a "wuss" for quitting mind you...we had people come and quit within a few months on a regular basis. All I can say is if in a few months you still feel depressed or broken down over how many animals are homeless, stuck in small cages, abandoned...you might want to reconsider working there. It's not going to get any better. I could go on a whole rant about how I like kill shelters better than no kills (or at least a hybrid between the two), but that's for another thread.
Just remember to look at the positives. Do adoptions - being on the adoption schedule was hectic, but totally worth it to send animals home. Do spend 10 extra seconds to give the animal you're working with an extra hug/scratch/pet. Do walk around with treats in your pocket and surprise your furry buddies every once and a while (make sure you work on training! Sit is a must for shelters, almost every customer will "test" sit with dogs). Do think of all the pets you've saved from being abandoned on the street and realize they're in a roofed building with food, water, and bedding for the time being. Do spend your lunch breaks outside with a hyper active dog and play fetch with him/her rather than sit inside, I would eat lunch in under 10 minutes and spend the other 20 outside with some dogs to fetch/wrestle around with. Smile at your customers, even the ones you hate, realize that in the end they just want a pet...even if their definition of pet or companion is not the same as yours. And don't hate every single person that abandoned their animals; some of them are *******s, but a lot of them have nowhere else to turn and are desperate.
If you waste all your energy being frustrated/sad/depressed, you won't have enough to be positive and have fun.
Sorry for the novel. Hope this post was somewhat helpful if it didn't scare you away, haha. I promise I wasn't a robot or anything at work, I did have fun although it was horribly stressful. You just need to learn to allocate your emotions/feelings. It's no different than vets really, who have to find a balance between caring for patients and not getting wrung up if one dies/gets sick/worse, etc.