I'm thinkin' he's a little jealous as much as he's not wantin' to admit it.

He's got a fair reason to be jealous a' either a' us, after all...

[Though he's really nervous now, he probably told her everything that went down with him. He's fearing the worst...His voice is rather shaky and it sounds like he's really only trying to convince himself that that's what the true reason Karkat doesn't want the two of them together in any sense.]