Why do men cheat

Something I’ve always wondered my fiancé and I have been together 3+ years and we are so happy like I wouldn’t have known if I haven’t had seen a muted conversation he had going on with some girl, (mind you it was recently) who he talked to before we were together. the thing I don’t understand is that I think I am good looking I cook I clean we have sex almost every day hot dirty sex sorry for the TMI and I give him all my attention and then I find this shit him calling her baby and I missed when we used to do this and remember that etc.i just wanna know why we had it so good and not in lost and confused! I don’t know what to do.