After do something which we regret,we always say "If we didn't do it……"Why did we do it precisely??
Because we are humans and always think about possibilities and something haven't happen. My personal opinion is we think this was because we want to escape present moment which seem to hard to deal with. So rather we think about think we should/shouldn't have done (instead or go and deal with it and make our lives better). I always try to don't think a lot about past. I do it, therefore, I cannot change it and this is the reason why I try to focus my thoughts more on present moment or future.