There were several reasons why Natsu was on his way to Lucy's house. Some were more practical than others, like he was hungry and she always kept her fridge stocked just for him. He also missed her. She hadn't been to the guild in nearly a week straight and he was beginning to get worried.
The only reason he had waited this long to pay her a visit was simple, she was mad at him.
She really had good reason to be, he supposed. What, with his burning of not only her favorite skirt, but also a hefty portion of her desk along with her stories. It wasn't his fault though. If Grey hadn't frozen his pants as a joke just seconds prior, he would have never needed to use his flames to melt and dry himself off.
Needless to say both he and Grey got an earful after this little incident and they were banned from the apartment until she saw fit to forgive them.
Natsu ignored this at first, of course. But he decided to give her some space when she practically threw him out the front door. So the third reason why one might find Natsu outside her apartment was to make an apology.
He didn't see why he needed to at first, but when she stopped coming to the guild, Erza banned him from returning until he properly apologized.
He left the guild grumbling but as he made his way to her apartment, he thought about what he was apologizing for and actually started to feel guilty.
As Natsu made his usual entrance through the window, he noticed that the lights were off. Seeing as how the bed underneath the window which he entered was empty, he made the assumption that she wasn't home.
Now he was faced with a decision. Should he stay and wait for her to return home? He wasn't even sure when she left or where she could have gone so waiting could take a long time.
This left him with his other option. To raid her fridge and go out looking for her. Natsu already had his mouth stuffed with a loaf of bread by the time he made his decision.
Alright this is it. I have no idea where to go from here.