I usually play with myself because other people suck to much to be in my group/party. Of course, this doesn't hold true for everyone, but if a game is meant to be fun, why dial down the fun because someone is just really, really bad.
Although, there was this one time I played team deathmatch in UT2K4 with my roommates when I lived in Virginia. But my teammate was too busy talking to some chick rather than playing so it was essentially a 1v2. We lost 32 to 40 but I still scored 38 points (yes, 38 > 32 if that says anything

). So this case it was fun playing against people but sucked playing with someone.
When I play tennis, I usually just play by myself practicing serves or whatever. It's really no wonder my arm is always so shot

.