Because I felt like it.
We all know that Dib doesn't really have any friends. D: Poor Dibbers. Let's cry for him.
Deesclamorr: I don't own Zim or Dib. But I own Zim's squeedlyspooch. Don't ask me how. -shifty eyes-
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"...Dib?"
"What?"
"Why are you sitting with me? WHY?!"
"...I don't have to answer that question."
"Well, why not? It was a perceptive question and Zim...ZIM DEMANDS AN ANSWER!"
"Why should I tell you?"
"Why not?"
"I can think of many reasons why not."
"Oh, FINE. Stinking humans...YOU'RE ALL ASKING TO BE DES--"
"Fine!"
"What?"
"It's because I don't have any friends. There, I said it--oh, what now, Zim?"
"...pff...ha...WAHAHAHAHAHAAAA...y-you...you're such a LOSER, Dib!"
"...Yeah, well, I don't see anyone hanging around YOU."
"...-snort- That's because I...do not wish to...intercommune with your race!"
"...At least I had a competent reason."
"You still don't have any friends."
"And neither do you, alien. So shut up and eat your sandwich."