Sounds like what used to be your hobby has become your job. If you spend your whole day sitting behind the computer and then go home and spend your off hours behind the computer, you run out of time to do anything fun (hang out with friends, read, etc.)

I am a big fan of golf, and when you look at the PGA Tour most of those players go on vacation and fish, or ski, or do some other activity - anything but golf. Golf becomes a job when you play at that level. I think a similar thing happens to good developers when they start developing full time for a company.

If you have a great project idea, try committing one hour a day to it when you have free time, and see if it really starts to turn out how you planned. If you still feel motivated to work on it, start working on it 2 hours a day - if you can make money from it, turn that into your full time job :)