It is True, drunk people usually end up doing something stupid.
The other person could be peer pressuring you and an internal factor could be a chemical in your brain
It really depends on the situation. In most cases it's because you've emotionally attached yourself to a person and/or opened up to them. It also depends on the reason for the breakup. If you feel as though the relationship shouldn't have ended and you're still in love and/or still happy with that person you may question why they didn't want you, putting faults on yourself and making you look at yourself differently which can cause symptoms of depression or anxiety. Etc.
