If someone told they couldn't imagine being happy doing anything else then I would honestly question their maturity. True happiness is intrinsic--it is not based on what you do or your situation in life. It is only influenced (and can be profoundly so) by those things. Long-term happiness is really far more dependent on your outlook on life. Two of the happiest people I know are one of our clinic receptionists, and one of the janitors at our VA. We all know people like that. Certainly the job we do can affect our happiness--it just doesn't (or shouldn't) determine it.
If someone really can't learn to be happy in another job, then I'm not so sure they'll really be happy in medicine. None of us really knows what we're getting into (for any job) until we do the job itself. Every job out there has annoying parts about it. You also learn that there are so many things unrelated to your job that have likely more impact on your happiness at work than your job itself (your coworkers personalities/outlook on their job, your boss, your patients/clients, whether your computer keeps rebooting on its own, etc.) These are all things none of us really think about when we're teenagers thinking about the career we want for the rest of our lives.
When I was a teenager and college student, I mostly was interested in jobs based on the idea of what I thought the job was (I can say the same about the girls I was interested in back then too). I did some shadowing/internships (for jobs--not girls!), but that's still not the same as doing it yourself. Still, it's the next best thing.
Choosing a job is similar to choosing a spouse in that it can certainly impact your happiness quite a bit, and you want the right match/fit. However, we get to date our spouse first. You don't actually get to date medicine--living with it and actually practicing it daily.
So there are just so many variables that a leap of faith is required. With medicine, that leap is just larger than most, as you're investing so much early on, in the form of time/money/hard work/delayed gratification. In addition, it is harder to leave medicine as many accumulate so much debt that they may not be able to get another job that pays enough to pay down their debt, whereas it's often easier to leave other jobs if you don't like them. Also, in medicine, some people get so used to/dependent on a six-figure lifestyle that they (or their family) are afraid to downsize, once again making it harder to leave.
There are plenty of other jobs I thought I'd be happy at, and would likely still be really happy to do. Park ranger and high school or community college teacher were at the top. But when it came to what would I like to do with my one and only life, as well as do for that entire life, medicine felt like the right fit.
I encourage people to view medical school and residency as something they should be excited about. They are the start of your medical career. If med school and residency don't excite you, then you need to ask yourself if it's really worth it. That's over 7 years of your life! I'm not sure there's anything out there I'd be willing to suffer 7 years for.
That ended up being longer than I intended. What it really comes down to is getting the best idea of "is medicine for me or not?" Then you take a leap of faith and go for it. You then put one foot in front of the other, and see where your path takes you.