Even though the way his eyes light up whenever he sees candy, or the sky being blue and clear, whenever he points out to you just how green the grass is in the fields, or how colorful the flowers are in March; whenever he gets to pet a cat or a dog, whenever he looks at anyone he loves, but you don't see the same look when he glances at you... it hurts you. It's not a very sharp pain, more like a load pressing on your chest and what feel like the symptoms of a sickness incubated in your stomach. All of it escapes every law of logic and reason. Why should you feel sick every time you look at him, every time he smiles at you but every time he smiles at someone else too? And why should it sting when the smiles he gives you are not like those he gives someone else?
For example, Haruka-senpai. Why does he get hugs, so many head nuzzles, so much respect and awe? Why does Nagisa look at him like he was the most beautiful creature he has ever seen? Why does it burn in the pit of your stomach and why do you care, why do you not seem to be able to push all of that discomfort out of you? Haruka-senpai is, objectively, a very beautiful boy. The way he moves in the water reminds you of the most graceful Betta fish, with the speed of a Marlin. So why does it hurt? Nagisa-kun's admiration makes perfect sense, why would it bring you pain? At first it was a light kind of longing as you looked at it from the outside, like some irrational nostalgia of a place you've never been, but then it began making you clench your fists and bite your lips in frustration.

It's unpleasant, it doesn't make any sense. It does not fit into the orderly grill you see the world though. The most beautiful things you know are those that make perfect sense. Statues with correct proportions, bodies that are seven-and-a-head tall, symmetry in a face's features.
The way two and two make four. No matter what, no matter the events and the circumstances, the answer is always four, and once you know it, it's simple to pile more and more calculations which are always correct until you climb the highest numbers to the point of giving yourself an headache, but the fundation will always be strong and reliable, if you lose count you can go back at any point of the addition and pick it back from there. Variables can be controlled too, they can be understood and foreseen. For example, chemical reactions within people's bodies can be regulated and when they are irregular they can be corrected. But why then do these reactions within you become so strong that you cannot just ignore them and blame them on hormones? Why do you catch yourself in wonder at how Nagisa-kun looks so perfect, even though he dresses horribly and his eyes are honestly too big to match any natural proportion? Why are you not bothered by the way he seems to not be able to mind his own business once? Why are you so naturally inclined to worry about him? You have noticed that about yourself lately, about how natural it comes to you, even though that boy can be such a nuisance. He is loud and disturbs you when you are studying or paying attention in class, to things you are genuinely interested in.
He puts himself in trouble constantly, and yet your chest tightens whenever you see danger incoming for him. Well... that's only natural, isn't it? Members of a species tend to look out for one another, as the pack needs numbers to be safer, and the Homo Sapiens Sapiens is definitely a species that evolved to be among similars, in groups.  But the way your guts twist whenever you think of Nagisa-kun getting hurt is not the same tug that pushes you into action whenever anyone else risks to be hurt. When it's Nagisa-kun, 'preserving' is an imperative verb in the most absolute, unquestionable way.

Why does it sting so much when he talks about 'Rin-chan' here and 'Rin-chan' there? Everybody is concerned about their friend. You don't know what went down between them and that drives you crazy. What was Rin-chan-san like, 'before'? He must have been very different to provoke such unanimous reactions. And yet that is an unknown element. You need to ignore it and solve everything else before it. That is how you solve equations, after all.
But swimming with these people, all of them, as dysfunctional as they might appear to be, brings you happiness. They happen to be quite odd individuals on their own, and yet when they come together, when they snap back and forth with their quirky conversations, when they calm each other down, they feel like they are all mixed together in one perfect masterpiece. They just work, they grind into place, they form their own grid through which the world appears to feel right. That is beautiful. Is there a place in there for you? You know there is. You felt it once, and the sensation of peace it brought you was so absolute you want to be immersed in it again.
Which is why you need to understand Rin-san too. Why his choices are so erratic and why he is making everybody so worried, Gou-san first of all.
And then it all falls into place. The reason why Rin-san was so full of bitterness had nothing to do with Haruka-senpai or anyone else. Only himself. The reason why he seemed to be in such constant pain. And you look at him lose miserably, then try to come out of the pool and fail, plopping back like a wounded animal. There is so much raw pain and anger you car barely stand to watch it, and you realize... it involves you as well. Your worrying, your investigations in the matter, the conversations about it with your team, you were part of it too, so why were you feeling so concerned about being excluded? When you gave Haruka-senpai your opinion on what freedom is, he listened. He even appeared to be surprised by the outlook you have on him. Others surely have ways to see you that are different from the way you see yourself. It is all so complex. Relationships and other individuals, the way they see their own world, overlap in ways that seem to be incompatible, to the point where it has an impact on the way you look at the universe, it crashes through your perfect grid. It is scary, but real nonetheless.

Could it be that this is how things are? How do things as illogical as feelings lace together and help you make sense of your world, as messy as they are?
Could it be that really, the way you think of others looking at you does not correspond to truth? Could it be that Nagisa-kun's different looks to different people and you are not out of him seeing you as something less?
Could it be? Could it be that these pains in your guts, these stabs and stinging needles going through your chest, that constriction in your throat is what they call 'romantic attraction', as silly as it sounds?
After all, if you think of this as an unsolved problem, though, you would have to do something about it, you would have to be proactive.
You remember of the way Nagisa-kun beamed as he told you he thought you were beautiful. The value you hold dearest. And you remember of his trust, that he is not afraid of doing reckless things because... he knows you are there. You remember of how comfortable he looks around you, and how he teases you, like a child who... oh. After all, if he didn't tease, then he would not care. The fact that you are his favorite target might mean more than the fact that you are more amusing to poke at than others. But it's like solving an equation by having two possible replies. You need to exclude one is possible. And in your case, you know it is very much possible.
Fear is not part of this process. After all, scientists would not come across any result if they had to be sure of the outcome every time. And even if this is not a scientific experimenti, it is true you would keep doubting for who knows how long about how Nagisa-kun really feels about you.

Yes, you suppose you are going to tell him, after all.