Imagine if Kanda was actually nice to someone. Lavi would just be staring at him and asking him if he was feeling alright and if he'd eaten something bad XD (either that or he'd get super mega jealous and be like "but you aren't even that nice to meeeeee" *pout pout*)
Kanda would grumble about annoying idiots and shut Lavi up with a kiss to get him off Kanda's back.