The Zeki Kiss was a big part part of chapter 46 in the manga, but I have no idea as to why they didn't add that scene in the anime...You guys have any idea why?

I'm not sure but its really a shame to cut that out! I don't like how they changed that scene, it made Yuuki seem really.....horrible.

Maybe it was because they're not planning/weren't planning on making a season 3, so to slow the story down they changed that scene a bit so it would be less dramatic, easier to close the series on. But it was a real disappointment when I saw they left it out :'(
Wish they kept it, ZeroxYuuki 4ever!♥