Why is my boyfriend so paranoid that I'm going to go off with another man?

Anonymous
He's got his heart set on it that I'm going to go off with some new man. I keep telling/reassuring him that I'm not and how much I love him and would never dream of going off with someone else, now I'm worried he's going to go and cheat on me because he's drilled it into his head that I will. He's a dj and sees a lot of stuff happen during the night also he's had previous ex gf's cheat on him. Every time I tell him I'm not like that and wouldn't do that to him he comes back with he trust's me but not other people. How can he trust me but have his heart set on me cheating on him?
Why is my boyfriend so paranoid that I'm going to go off with another man?
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