I'm pretty happy, approx 5 years out of residency. I chalk it up to:
1) I didn't ever "fall in love with medicine." I always looked at medicine as just one option among many, and I have always seen medicine as a job. It's a lot easier to be satisfied with your job than it is to be satisfied with your "calling" or your "destiny." There's far less potential disappointment, when you realize that most shifts are just moving the meat. Sure, it's a pain trying to satisfy a bunch of entitled patients (and administrators), but there is no magical job in which you are spared from interacting with *******s.
At least I don't have a supervisor looking over my shoulder all day long, trying to micromanage every decision I make. THAT's what I wanted to avoid in a job, and in EM, I've succeeded. Happy!
2) I like my job because it offers the possibility of continuing improvement and it is difficult to to completely master. I'm a much better ER doc now than I was when I graduated, and this feeling of improvement makes me happy. Every month I can pick an area of my practice and focus on improving it. That part of my job is fun. Since I graduated, my focus areas have included a) dealing with consultants that feel it's OK to act like *******s, b) central lines and intubations, elegance above mere competence, c) diagnosing abdominal pain with H and P, before the imaging comes back, c) efficient, speedy, and effective charting, d) the neuro exam. In the near future I'm going to be focusing on regional ultrasound-guided nerve blocks.
I'm a proponent of Cal Newport's "craftsman mentality."
http://calnewport.com/blog/ Forget passion -- job satisfaction comes from autonomy and mastery and stuff like that. EM is ripe with these opportunities.
3) I am happy because I am not an EM doc. I'm a person with varied interests who happens to practice emergency medicine as a career. I like EM, but also politics, athletics, certain kinds of novels, legal issues, financial policy, environmental policy, etc. I do other things besides EM, and in any given month, these things may be (often are) more important to me than EM.
4) I'm happy because I'm not overcommitted financially. Well, sure, I've got humongous student loans that I'm working to pay off, but my monthly financial obligations are easily covered by my professional income. I didn't run off after residency and buy the biggest house I could talk my way into a loan for. I'm still waiting on the M3 BMW I've lusted after for years. I don't have a cable bill, 'cause most of the good stuff on cable, I can watch a day or two later on Hulu or Youtube.
5) I don't work too much. Too much work makes me miserable, so I avoid doing that.
EDIT: oh, and coffee. Lots and lots of hot, black coffee.