You're not supposed to get rattled in poker; you're supposed to remain poker-faced!
For people that are degenerates, if you've spent so much time feeling a certain way, it's actually uncomfortable to feel like a winner. The familiarity of losing is, in an inverse way, comforting. At least you know where you stand.
With gamblers, it's never enough. You keep thinking on a conscious level that the prize is whatever the pot is. In poker, you may have a certain goal for the day. But the truth is, as soon as you win, more often than not the first thing that comes to mind is: If I'd bet more, I would have won more.