It depends on who I'm hitting with. I practice a lot with top level guys, so I don't mind losing to them as I really expect to take my lumps from them when they heat it up.
That being said I now despise losing to people at my level. It drives me up the wall. I used to play only for fun, but now that I'm able to hang with guys who are better, it kills me inside to lose to people at my level.
Stupid mentality? I think so. Does it motivate me to work harder and make sure I don't lose? Yes.
To me now, losing sucks and I definitely go through the replays in my head. I have a check list that I use religiously after every time I have a match on what I did, what I didn't do, and what I can do to improve that. It's a bit extreme, but it helps.