why do boyfriends cheat.

idek why i'm making this post bc im too upset to even go into much detail but why guys always cheat no matter how good u are to them? like my bf has cheated on me before and now my friend told me how see saw him hanging out with his ex today at the movies when he told me that he was taking his little sister and he said he just ran into his ex and that's all but i don't buy it. he's lied and cheated in the past, what would stop him from doing it again? idk. he said he needs to talk to me otp/in person tomorrow bc he "wants to tell me something that can't be explained thru texts" i feel like that's just him trying to buy more time to come up with an excuse or something. anyways we'll see what he has to say tomorrow but should i just be done with him this time?