He didn't mean to really. More like he wouldn't dare laugh at Katniss when they were doing something so intimate and new to the both of them (new in this context anyway). He was just happy and still in a bit of disbelief that she would be okay with this, that they were even doing this. Was this considered "making out"? He wasn't even sure.

The use of her teeth definitely caught him off guard however and in a split second he was the one left making a pleased sound, the hand that was resting on her back curling against it instead. That had felt... really nice, better than he was expecting, and it pushed him back into line, lips slightly parting in reaction and kissing her top lip.