Prior to surgery I had read a lot about the potential for post-op depression. So any time I felt even the slightest hint of depression coming on, I recognized it and made sure I kept myself occupied to get out of it and thankfully it worked every time.
I think part of the problem (just my theory) is that your mind is so preoccupied prior to surgery with surgery that once surgery (and possibly recovery) is over, your mind has all this empty space that needs to be filled. I mean for me, the months leading to surgery it didn't matter what I was doing, eating, working, driving, sleeping, watching tv, whatever, surgery still still weighing heavily on my mind. But once it was over, that mental preoccupation gave me nothing huge to look forward to.
I also believe (again just my opinion), is that you get this thought that after surgery life is going to be perfect and all of your problems are going to disappear, but they don't. The bills are still due, people get sick, the car breaks down, etc. Life will still have it's ups and downs, just your genitals will be different.