It's complicated. Just imagine several requests for the same league, for example. You are thinking about two friends trying to play together and have more fun, and that's a legit reason, but with this possibility, we will probably have lots of request from experienced users trying to play in an easier league. And you just have to get on well with another guy in the game to have that. It would end in a race to see which team sends the request first. So the real friends will end in different leagues and more frustration.
As a start point it's not a bad idea, but needs more elaboration.