When it happened, Gray was sleeping on the couch with Natsu in the room and he started muttering things in his sleep.
"Natsu-kun! I love you!" he said. "Please never leave me alone!"
Now, Natsu was just confused. He thought that Gray had hated him, or at least disliked him. It was very surprising to find out that he... loved him... Plus, since when was he gay anyways?! Natsu wasn't sure how to react to this, but for some reason he was slightly pleased to find out about it.
"What the hell am I thinking?!" thought Natsu, "I'm not gay, and even if I was I wouldn't want to be with that asshole Gray!"
But, as he gave it more thought, and listened to more of Gray's sleep-talking, he realized that maybe he did have feelings for the other wizard.
Somehow, now that he finally decided to listen to his own emotions, he felt like the sudden love for Gray was overwhelming. He didn't understand what it was like to be in love with someone, let alone a guy. Not only was he another man, but a guy who he thought he hated. Was it really okay for him to feel this way? Would the others in
Fairy Tail judge him if they found out? Concluding that they probably would have a problem with it, Natsu decided to keep it a secret for a while. Maybe he would try to tell Gray sometime soon, though...
This was all so new to him. He had never really felt passionate about anything other than becoming a powerful wizard and being reunited with his father Igneel. Suddenly, this person who he thought he hated had become the object of his affection. How could that happen so quickly? Well, the reality was that he was actually in love with Gray for a long time, but refused to admit it to himself. That was his way of protecting not only his pride, but his emotions. Being in love could cause all kinds of pain; the kind of pain that affects your mind instead of your body. That was truly the worst kind of pain.
Gray had actually realized that he loved Natsu several months before this event, but he had kept it a secret and continued fighting with Natsu to make sure that no one would figure out his true feelings. There was too much worry that would come from the other members of Fairy Tail finding out about this. Not only would they scrutinize him and Natsu, but it would also embarrass all of the other members of the guild when the public found out about it. After all, they were already criticized for being somewhat reckless, and two male members being romantically involved with each other would just create even more controversy. So, for the sake of the group and of their own emotions, Gray and Natsu both decided that they had to keep it a secret. At least for a while. But maybe they could tell each other sooner, or at least Natsu could, considering how he already knew that Gray felt the same way.