Natsu and Gray are fighting since they were childhood friends. They were called rival friends. Also their powers are opposites like Ice vs Flame یا Fire. And it's like they are دکھانا off their powers. Like who's the strongest! Natsu یا Gray! Like that. And for those who believed that their fighting because of Lucy then go don't stop believing cause everyone of us can have our very own version یا opinion. And that's how I put it to words. Usually they are just دکھانا of their powers.
Natsu and Gray fight because, I think, that their Magics', آگ کے, آگ and Ice, are opposites. And as they've known each other since they were children, they've become somewhat like brothers, and, like brothers, they quarrel.
I dont think it is Lucy because they both are always fighting even from their childhood. Its like friendly rivals because they do help each other a lot . may be to prove that himself is مزید stronger.
Natsu and Gray always fight is because آگ کے, آگ and ice doesn't go together and they always fight when they were little, so yeah it's now because of Lucy یا strength