Friends with benefits getting angry and jealous why is this?

So I have a friends with benefits which I’m very happy with casual however recently I’ve noticed he’s started getting bothered by things I say and starts an argument. He got upset because I jokingly said I was going to replace him and has kept bringing it up that I apparently want someone else we didn’t speak for a few days after that argument and he messaged me saying how he doesn’t want to argue and doesn’t want to keep going round in circles. He has been asking me if I have met anyone else and all this aswell
Friends with benefits getting angry and jealous why is this?
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