Her scent calls to his beast. Her voice drives him crazy. No one can stop him from claiming what's his. Mating fever is upon him and he will make it so that she knows who she belongs to.

Natsu and Happy use Lucy's panties as masks to play "Ninja" and Natsu notices how nice they smell. Sniffing panties isn't the best way to win a girl's heart. Nor is stealing them. Nor doing naughty things with them!