I don't really think of myself as ever "re-starting". But I keep making new saves whenever I have new ideas about something I want to try, or a set of sims I want to make, or a story I want to play. I don't quit playing the old saves when I start new ones, but gradually over time my focus drifts away from older saves as newer ones take up my attention. So it has the same basic effect as re-starting does, which is that I'm often starting over in terms of the sims being new, not having much money or skills, etc. But all the saves feature entirely different sims as the main household, so none of them are actual re-starts of previous households.
In other words, I don't start new saves because I got bored with the old ones, I start new saves because my creative idea generator doesn't have an "off" switch.
The reason I gradually drift toward the new saves it is probably just because new projects are intrinsically more exciting to me than old projects. It's like that in all parts of my life. Not just Sims.