There are a lot of good reasons to sell a company but one of the least obvious is being too successful. Success changes people, it can make them believe they are super-humanly clever or impose an uncomfortable "imposter syndrome" where they feel they're actually only there by luck and are sure to be caught out soon.
There are, of course, many other effects but this letter from Minecraft founder Markus “Notch” Persson suggests that sometimes, success just makes you miserable.
It’s not about the money. It’s about my sanity.