In my experience I've seen the same situation happen over and over again:

Attractive and nice woman has problems with her boyfriend (name your typical relationship problem). Woman starts to have hypergamous instincts and allows new men to flirt with her and/or maybe confesses her problems to another man.

New man lends an ear and hopes to give advice, but also starts to talk himself up in ways that her boyfriend was lacking. New man seemingly doesn't care that the woman may be jumping ship but rather wants to swoop in and be the solution to her problem.

Why don't men seem to care that a woman leaves her partner for a new man, instead of seeing how loyal she is to him?