Because people are bored, and because most people finds it fun to flame others.
That's just my opinion about how a flamer would think though O__o
Edit: Some definitions from urban dictionary on what a Flammer is xd;
1. Flammer:
Someone who acts gay and is a wanna be
"Get away from me you ****ing Flammer"
2. flammer:
A word used by morons who fail to realize that they mean 'flamer'.
<flammer> omg u r teh flammer
<Flamer> And you're a grammatical reject.
3. flammer:
someones whos a druggy
go see if you can get some from him
hes a (flammer)
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Some definitions from urban dictionary on what a Flamer is;
1. flamer:
Extremely flamboyant homosexual. Very bright clothing, loud speach, upbeat attitude, often noisy or annoying, intrusive. Very very gay...
"That guy is acting out some broadway musical in the middle of the street.. what a flamer.."
2. flamer:
1. A flamboyant homosexual.
2. A person who deliberately makes inflammitory or slanderous posts on internet message boards for the purpose of starting a flame war.
1. That one dude from Will & Grace.
2. The forum member's tendancy to insult and pick fights with other users earned him a reputation as a flamer.
3. flamer:
1. An outgoing homosexual, who is very open with his gayity.
2. A person who constantly starts fights on forums or message boards. Usually with many vilgar statements, and outright lies.
"That guy is ALWAYS insulting my posts, no matter what! He's such a damn flamer!"