Well, I never got upset playing Falco. If anything I think he's underrated. But playing Greninja makes me so irrationally angry.
I think part of it is that some characters are a
lot of work to play right. There are times where I feel like you are really maximizing Greninja's potential, and you are hopping and fast-falling all over the place with shurikens and perfectly spaced Fairs. Then there are times when you just run into Shadow Ball three times and die in an attempt to go for a dash attack
I totally understand this, because I do the same thing with Mewtwo. It's just exhausting to play him well, because my hands have to be doing so much work, and no matter what people say, the Gamecube controller is not good for this much continuous action. The continual perfect execution needed to masterfully maneuver a character is too much for me, really. Maybe you get angry because you get tired, and when we get tired we become more irritable.
Just a thought. I know that while Bowser isn't very exciting to play these days (potential feels tapped out, very linear in style), I can play him well for long periods of time because he isn't very taxing on my hands.