Let me rephrase it, internship is a breeze compared to the BS of MS3 year. It's the power of a positive attitude. No more chasing down a physician to write an order, countersign your charts, scut you out. You work harder than you probably ever have, but you control your life, your orders and the direction of your patient's care. That makes any "bad" rotation much better because you are a physician rather than a poor student.
I did a transitional year. With that in mind, my friends in IM/GS were much more frustrated. Peds were happy. Psych were quirky. OB/GYN, well, living on the labor deck. We worked the same hours, had the same call schedule, but were a happier intern group as a whole, as we were owned by no single service for the year. No rivalries/competition and we were a tight group picking up for each other when overloaded.
As for depressed, we had one guy stop internship for a couple of months due to SI. Another dropped out all together, after being found non-functional/incompetent. Another who was gung ho IM, dropped out late in PGY2 because she couldn't handle the stress of being a resident and a single mom. This is out of 85 interns, so, health care professionals do have real problems during training. Recognize your burnout/depression and deal with it properly. Don't gloss it over as a weakness or a shortcoming.