I think I find myself somewhere between Milkman and mmmcdowe. My happiness has been entirely class-dependent. During biochem/molecular bio, I was happy (working hard but still having a life, getting to know my classmates, getting a lot of shadowing in, etc. During anatomy...I've never been so miserable in my life. After 4 months of it I was absolutely ready to quit. There were a number of factors: I hated the class (I have zero interest in it, and I'm awful spatially), had one horrendous person in my lab group that made lab awful, never had time to go out because I was always behind, and performed poorly on the tests even though I was putting in tons of hours.
After anatomy ended, and micro started, I became happy again, and the past couple of months have been awesome. I've had to study hard, but I performed very well, understood the material, enjoyed it, and got a lot out of it. Now it's the summer and I'm doing research and it's a blast because most of my classmates are still here.
So yeah, it's been a roller-coaster ride. No, med school isn't inherently miserable. You have to get a lot of pleasure out of learning to enjoy it, and have to not mind putting in the long hours. Some classes will work for you, some might not.
The main thing I want to say is that it's ok to be unhappy.Med schools often are filled with either fakely rah-rah people and people who feel they have to seem happy and in control. There's nothing more dangerous than being unhappy and still pretending everything is great. If something isn't working, admit it to yourself and tell someone (friends, counselors, whatever). It's funny, the day that I admitted to a friend of mine how much I hated the class we were in, he admitted the same thing and others followed suit. It was nice to get it off our chests and we helped each other get through it.