i don't get it

I think it's so stupid when girls/guys talk shit about a guy/girl they had a relationship with. It's stupid because if there's a chance for them to get back together again, they get back together. But I thought you hated her/him ? I thought you would never get back with her/him ? I thought you're too good for her/him ? I also think it's stupid when girls/guys think it's okay for them to cheat, flirt, hurts others, etc, but when it backfires, they get super pissed & emotional. What makes you so great to do that to others, but noone can do it to you ? Explain To Me. -o-;