Thing is, to be good at *any* martial art, you have to be relaxed. The more wired and tense you are, the harder everything is. When you get excited/scared/angry, your field of vision narrows, your muscles tense, your movements become jerky. Not only is this bad for performing a technique, it also gives your opponent something to work with and use against you.
I take two martial arts. I've been in eskrima for about two years, and aikido for about six months. They're both very different: one is very hard (aggressive, bone and joint breaks, weapons, etc.), the other is very soft (revolves around not hurting your opponent any more than you have to, almost entirely defensive). I love the balance this brings.
Another bit - martial arts are a spectacular way to blow off steam. I've got an anxiety disorder, and giving myself time (and confidence!) to beat things with sticks or toss 300 lb men around like they're made of paper is exhilarating.