What I'd give for a girl like Yuno(not exactly as crazy maybe)..... all it takes is something like a "I love you" to keep her at bay. lol.

Anyways, it's probably the fact that Yuno is a nutcase that Yuki probably has 2nd thoughts about her. Wouldn't you guys be scared? Yeah she is crazy about you, but you have no idea when she might just snap and kill you as well.