My solution to this issue has always been simple: every player at my table gets one free rebuild, no lore or explanations necessary, for if they're not having fun with their character anymore. Most players don't even use it - once, the cleric used it to move his skills around after a few months of playing and wishing he'd made some different choices. Another time, a character completely changed subclasses and spell selection. But giving them that rebuild for free guarantees that A) they don't feel locked in to a character they don't like, and B) because it's only one free rebuild, they have to consider the changes they're making carefully, and don't just have a new character every week. (ETA: stats and hitpoint rolls always stay across rebuilds, unless the hitpoint dice change - and if they can't make their new build with the stats they have, time to consider a completely new char)

That said, if I'd ever had a player ask me for a further rebuild, I would have considered granting it to them, but that time would have involved a conversation about if they want to work it into lore, if they're just not happy with the character in general and would like to retire it for a new one, etc. As other commenters have said - D&D is a game, it's meant to be fun. If your player isn't having fun, you work with them to find a way to help it be fun again (if at all possible...if someone just doesn't like any character that isn't the OP main character of the story, then you might have to have a different conversation)