Stress
When you're dealing with other stressed people, even small ones, there's bound to be stress on your side as well. You must be able to maintain your professional detachment through heartbreaking revelations (although you may want to keep a teddy bear at home for emotional support).
It can also be hard to decompress after sessions, because you can't gossip to others about the juicy details...pesky doctor-patient confidentiality. There may not be anyone around to talk to, period, if you work on your own.
Home life can also be tricky. Your spouse might understand why you can't go into detail about your day, but that won't improve matters when you have to take off to treat a patient in crisis.
Another stressful aspect of being a psychologist is that it's not all about the people. You might spend a good portion of your time wrangling reports for health care providers, or compiling IEPs for troubled students.
This is on top of your regular appointments with patients, so get used to being over-scheduled. You may have to work evening or weekend hours to stay available to your patients, especially if you're in a private practice. That is, if you have enough patients in the first place...be sure to sock enough away to make it through lean months like the holidays, when your clients are all off creating fuel for future therapy sessions.
All right, all right, that's enough moping. Did we mention that this job can also be seriously cute?