When he says he wants to be just 'friends' but sending mixed signals?

Obviously, he likes me. But the question is, I don't understand why these things happen to people like me. Guys say they want to be friends, they are interested in me, they want to propose to me, buuuut they go on and move away from the relationship and wants to marry another girl? Like wtf man. What's going on? Because I think every relationship is moving too fast.
When he says he wants to be just 'friends' but sending mixed signals?
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