Even after Lisanna had returned, Natsu had a difficult time finding time to spend with his once love. It had been, what four years? Lucy took up so much of his time. After meeting her, he had near instantly decided to make her his mate, in light of Lisanna's disappearance.
But in the end it hadn't worked. Lucy didn't accept his dragon instincts. Not like Lisanna had done. And the only who even began to understand the situation was Gajeel, being raised by a dragon as well.
It was just too unfair to Natsu. The person he had originally planned on making his mate coming back to life and the person who was basically her replacement. Natsu was at a loss. So he decided to mark her. And she rejected him. The guild naturally jumped to her defense, and his inner dragon reared up to defend himself from the accusations.
It was a year after the fiasco when Lisanna approached him. Told him that she didn't hold his actions against him. But Natsu knew that was just because she didn't know the beautiful blonde.
During his journey with Gildarts, he learned about the world and societies customs. The very information Igneel had been incapable of teaching him and Erza had never educated him on. Slowly, Natsu began to understand that he had somewhat jumped to conclusions. It didn't make it any easier on him. He just started blaming himself even more.
After that one year, Gildarts knew that Natsu regretted his actions and let him go. The older man was proud of Natsu. He had understood his actions and took responsibility for them.
Gildarts didn't know that Natsu had snuck off and seen Gray removing his mark. He didn't know that Natsu still felt slightly possessive of the pretty blonde celestial spirit Mage. But once Gildarts released him, Natsu was far more mature.
It was also when Natsu found out about Lucy and Gray's wedding. He had just wanted to watch from a distance, but of course Freed had stopped him. Freed, Gajeel and Elfman. While Natsu may have been able to beat the three of them, he didn't want to draw attention to himself. Besides, it wasn't like he could have gotten past the runes anyways.
And all of that led to where Natsu was now. Standing in front of Lisanna's door.