How can someone take online gaming so seriously?

After two stressful games of "trolling" the Summoner's Rift with stuff that I'd never touched before, I'm under the impression that trying out new things is detrimental to the sanity to others. Let's just say that 20 minutes of people yelling at you for not living up to their expectations and scavenging experience points out of desperation in an losing match can bring out the worst in people. As in behaving in an manner that gives me the impression that your less than half my age. I'm perfectly fine with losing since I consider it to be an valuable lesson; except for the endless demands, which are entertaining to me. I'd see if you had actual money on that game, but other than that, why should I care?

Hard to say. Lots of reasons for it. Some folks just don't like losing, while others, once they think they've figured out the "metagame", get stuck in their habits and get annoyed when others don't meet their perceptions of correct play.

The high of victory. There's definitely more to it but imo that's the core. Being better than someone at something, and winning a game with your skills and crew feels great for anyone.