That's what kills me about tennis. There are a lot of people who play who are wound way too tight or seem a little mentally unstable. Of all the sports I play, this one definitely has a higher percentage of weird people.
I think it's probably because you're so lonely out there on the court. Every mistake you make, it's yours only, and if you run into a superior opponent against whom you just don't have a chance, it doesn't take away any frustration ;-)
Whereas in a team sport, you have others with whom to share emotions, and the burden of responsibility is more widely spread, too.
Oh, and tennis is different from other racket sports, such as badminton, squash, or table tennis, as there is so much free space around you and you really have noone else near you for venturing your emotions.
All in all, a great stage for ups and downs, and that is exactly what I love about it.
As far as for myself, though, on the court I never curse outwardly, throw my racket or go into a tantrum. But that's simply because a) I know I'm not good enough to justify such a behavior, b) I don't want to make a fool of myself in front of others (have a high degree of self-monitoring right there ;-) ), c) my equipment is valuable to me (I'm actually a little ocd about it) and I don't want to break it, and d) I know I'll find other issues in my life where I can venture my emotions.
But if my opponent does it, I really struggle to keep that grin off my face ;-) Other than that, I don't care. After all, he's making me look good in comparison. And I'm not there to educate other people on the tennis court.