This thread is actually the first thread on Boards I'll have posted in. I'm glad it's such a positive one. (Also, this will be a wall. Sorry, I'm used to making long posts when it comes to forums.)
Positivity and optimism like this is definitely what keeps me playing. In the end, it's all supposed to be fun, right? :3
So many (nowhere near as much, recently, though) people that I've played with have attacked teammates, or me whenever the slightest screw-up occurs. That tends to cause stress, and will only negatively affect my/others' performance. Some people could just be having an off day/game, or some could've just derped for once in a while, and those that attack them will just be causing their performance to go even farther downhill. Not to mention: how can you focus on attacking the enemy if you're already focused on attacking your own team?
In games where mainly your team (preferably both teams) is positive, chill, accepting of loss, etc, things tend to go much more smoothly and are much more fun. It personally allows me to truly play at my best, as I feel as I'm playing with friends instead of unsteady acquaintances who'll likely jump on your every mistake, or pessimists who'll give up after the first blood saying "It's over; surrender at 20." Also, pretty much every game that I've played that's turned around a very likely loss had positive teammates, as we felt we could make a good last stand before actually surrendering.
Finally, I wouldn't ever say that "becoming nice" would be a waste. You may not immediately see an increase in wins rate (or even ever) but at least you'd have a more positive outlook that'll at least affect your own gameplay, even a little bit, and allow you to better take a loss, as those will happen. In my opinion, any sort of positivity that you could have over the opposite is ultimately better for everyone; making the best of what you got (even if everyone on the team is like 0/69/0 or something, in which case I'd be too busy laughing to be remotely agitated) is always a better view to have.