1)they always go on a mission together
2)natsu will do anything to save lucy like the time when she was falling of the building
3)natsu didn't have any fun in the arcobaleno blossom seeing picnic.
4)natsu pulled off a albero just for lucy could see it
5)his dialogue:"just te try hurting a single hair on her head cuz i'll turn te into smouldering ashes" when she was captured.
6)when everybody was talking about natsu and another girl she was the one having that weird liking thought
7)in the oracioun seis arc when lucy and natsu were falling off the waterfall lucy held enough tightly to protect him