Because you can play thousands of good quality games,by people who are passionate about their work.And because you can make new friends,and feel wanted whenever you log on to the sight.
I love it for various reason. I love the Amazing Game content of all kinds from Kingdom Rush, to Music Catch, or even the insanely random Ignite People on Fire.
I also love the Ranking system, and AP's and Armatars, It helps immensely in my opinion in keeping people coming back to the site.
Another great aspect is the Forums, even though I wish they were a little more user friendly, and had a few more features.
i've met a lot of friends on here, the games are awesome, interesting forums, awesome armatars, good art, overall the best site i go on all the time!!! oh and the admins/mods are great at their job!