Natsu Dragneel, is a player who played with girls a lot. (Mostly sleeping with them)He started doing that ever since his girlfriend cheated on him and really hurt him. He met a girl named Lucy Heartfilia and he founds out that she is different from any other girl in Fairy High.
They started dating but he also has his interests with a girl named Lissana. He slept with her,but didn't had any relationship towards one another Lucy already knew that he was going to hurt her, but she wanted to know how far can he go to do so.
Lucy finds out that he slept with Lissana but she actually never cared what relationship they had, so she lived her typical life, ignoring Natsu and just be happy about her life.
Natsu knowing that she found out about what happened, he wonders why she hasn't yelled at him, or slaped him on the face for doing so. The next day, Lucy brakes up with Natsu, and doesn't want to know anything to do with him.
Both have feelings
(Don't own Fairy Tail. Hiro Mashima does)