Because it's a shit reason.
If you see an asshole in the morning, you saw an asshole.
If you see assholes all the time, maybe you're the asshole.
Seriously, you need to take a step back and change your perspective. Why are you getting toxic? because someone fucked up? Oh dear lord are you really going to expect perfection? Expect improvement, sure but understand that people learn at different paces and they may take longer than a game to learn or may not really adapt until the next game.
Try to help your team, not just by running over and saving them - that's white knighting it, and nobody cares for that. Don't be a hero, just be part of the team. People start yelling? tell them to calm down and play. I do that a lot. Have anger problems? I highly doubt it's confined to this game. Want to not be a toxic player? learn to be a better person.
And lastly, remember that there is no real emotion that is in text. You can't tell whether I am being sincere or sarcastic and your interpretation will likely depend on your mood when you read this. Maybe it's a bit of 'fatherly advice' but I don't mean anything bad to you or anyone else. Having problems in your life or your past is never an excuse for behavior - it may be a reason for it, but you can always change the way you act from here on forward. So, if you read this in any negative or harsh manner, I just ask you to re-read this in a calm and helpful way of passing on some life experience.
[slayer-pantheon-thumbs]