Because you can't regret what you haven't done yet. Before doing it, you're all about it. You're all about the desire and lust for whatever it is. After you do it, though, you feel guilty because those blinding feelings of lust have disappeared and now you see what you have done. You can't regret doing something that you haven't done.
because u get so distracted by whats goin on around you, and when you finally cheat or something you then realize what your life would be like without ur significant other in ur life. you may be getting bored or things getting old so you venture without even realizing it. but everyone is different. they may do it cuz they have been hurt so many times that they wanna make every girl they r with hurt as well.. jus depends on the person
it has to do a lot with hormones,a rush of adrenaline from something considered taboo ,feelings of self-entitlement, need for intimacy that they aren't finding in the present relationship. only some people regret it while for others it fuels the desire for more of it.