Ultimately, Natsu and Gray are completely different; from the elements they control, to their height and of course, their personalities. So it’s not surprising that they don’t see eye to eye pretty much on a daily basis. Natsu acts and speaks his mind without thinking, while Gray is more of a planner and is generally more reserved, except for the nakedness... Natsu’s hot-headedness tends to get on Grays’s nerves since he’s normally just chilling at a table and Natsu runs in screaming, usually insulting him and/or challenging him to a battle or some sort of contest.

Of course these clashing differences cause trouble in paradise. Though their bickering has become less and less over time, they tend to argue over the littlest things like the tone of Natsu’s voice or Gray’s inability to keep his clothes on. One time they argued and bickered for three days straight, only taking breaks to eat and sleep! They remind me of those couples who just couldn’t stand each other when they first met and then end up hooking up, and before you know it, they’re married!

Since they were children, Natsu and Gray have been like competitive siblings. Natsu is always the one to name the challenge, but Gray rarely declines. They both want to prove to the other that they are better in any way possible. We all know a couple like that, whether they’re romantically involved or just friends.

When the Fairy Tail members are reconstructing the guild after its destruction, Natsu challenges Gray to carry twice as many wooden beams as him, to which Gray complies, of course. (Spoiler: Gray does it but can only handle it for a short time). They even compete over how many bad guys they’ve beaten while engaged in large scale battles! And then there are the eating contests, a sand castle competition, swimming race, tanning... You get the idea. And then the random “my _______ is better / bigger than yours” moments. It makes you wonder if they ever compare… other things…

When Gray was about to kill himself, Natsu stops him and tells him not to give up and to stop running away. This changed Gray’s view on life and his friends, and it served him later on when he remembered his dear Natsu’s words, which helped him find an alternative way of defeating his foe that didn’t require him dying.

When they fight as a team, or even against each other, they can predict each other’s moves and know they can count on the right reaction from one another. Like during the battle against Mard Geer, where they both attack in unison and use their magic in perfect tempo to get things done.

Ever since Natsu and Gray learned how to get along better and fight side-to-side, the best part of their team work has to be when they’re standing back to back, Gray sans shirt and Natsu with his vest & scarf combo that allows for a nice look at his biceps and pecs. Then they give each other “the look” and proceed to kick tremendous ass! One can only imagine what else that look can trigger between the two…

Many times, when you’ve been with a partner you love and trust long enough, you start knowing what they’re thinking and words become unnecessary. With just a nod, or a glance, you know you’re both on the same page. Natsu and Gray are no different. There have been many instances where, with no words, Natsu will distract an enemy knowing that Gray will take care of the next attack or vice versa.

They were on the same frequency even before they got along! Before they were put in the same team by Erza, she had forbidden them from fighting, which of course they ignored and kept arguing and bickering. Upon Erza’s return to the beer hall, and with no warning, they both knew how to act and what to say so she wouldn’t know they were in mid squabble.

While they argue and bicker, Gray and Natsu care a lot for each other. From Gray recovering Natsu’s lost scarf after a battle and scolding him for losing something important (such a wifey thing to do!), to Natsu punching Gray unconscious to prevent him from getting hurt further by Lyon during their battle. Before which, Gray threw Natsu off a cliff to save him from Lyon’s attack as well.
No matter how much these two fight and say they can’t stand each other, they would never let the other one die or suffer if they could help it. They have been known to play pool together, nap together and they have saved each other’s lives multiple times. While they may not admit it out loud, we all know they couldn’t live without one another.