I don't understand why he said we can't be "just friends"?

I was friends with benefits with this guy for over a year and recently he ended things. But he says we can't be "just friends" when previously he said we could be without having sex. I don't understand why he and I can't just be friends? Especially since I'm dating another guy?

Like if we are both dating or involved with other people. Then why couldn't he and I be just friends? Like he said initially?

I don't understand why he said we can't be "just friends"?
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