For me, depending on the battle, I would get a little upset at myself for not doing such a great job, and feel I could do way better. But it's not that serious, because at the end of the day, it's just a game. Winning and losing comes with the territory, so I do not understand why many players find the need to berate others after their loss by sending them hate mail. It won't change the outcome of the match.
@Ichiron47 same for me! If I lose, I feel disappointed and a little angry, but I realize my mistakes, and I just go practice and try to improve on what I did wrong. Sending a hate mail message may feel good for that moment, but will it change the fact that you lost? No, so salty players shouldn't ridicule the victor for just winning like they were trying to do:-)