Restart itis is a great way to put what I have. There are lots of reasons I restart, but the main three are 1) Starting a new legacy and becoming generally unhappy about either the legacy starter or the subsequent generations 2) I do get to gen 3 or 4 or 5 (that's the longest I've lasted) and it all starts to feel a little out of control--too many siblings having too many kids and the rotational play just getting out of hand and 3) I'm very OCD about the worlds themselves. I'm not a builder but I do like to replace each and every lot with what I consider the perfect replacement for that lot. And as more and more of my favorite builders release better and better lots, it gets to a point where I almost want to start over to create the perfect worlds.
I do finally think I've solved at least #1. I created a 1st gen legacy starter sim I like so much that I only ever want to kick things off with her. I saved her to my library and uploaded her to the gallery just to be extra safe. (I must have done a good job on her because she's my most popular download). So far I've only restarted her once since I created her, and that was because of problem #3--the world wasn't perfect enough for her.