For serious now...
Destiny employs addictive gameplay reward systems. They are the same systems in use by the gambling and illicit drug industries.
Google how the brain processes rewards on a physical level. Then you will understand.
When people are used to the reward and its not given the body physically, in the real world, experiences withdrawl. This lowers the players ability to engage in rational thought and communication.
And thus, a toxic community is born.
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...wait, that's the reason? Huh. Well that explains quite a bit :)