It's not that Natsuki doesn't realize how important Haru is to Yuki, he really understands that. He understands that because he owes Haru in a way too, for introducing the both of them into each other's worlds. He understands that well, and Haru is his friend, and he does care about him, appreciate him.
But sometimes he feels that Yuki brushes a little too close to Haru, gets a little too kind or caring, and something in him is set on fire, and not in a good way, but in a way that feels both cold and hot at the same time.
Don't touch him. Stick with me. Worry about me. Talk to me.
Thoughts rush in and out of his head like rain, and he can't stop them - can't stop himself from jealousy and irritation. His desire to monopolize Yuki - kind, bright Yuki, who doesn't understand everything but can be everything Natsuki needs - is overwhelming.
He wonders if Yuki ever feels like this about him, wonders if Yuki ever worries that Natsuki will be swayed - not that he ever would be, but he wonders and deep inside, he hopes Yuki does. He doesn't want to be the only one caring. He wants this feeling to be mutual, so that they don't drift, don't wilt.
So when he steps in and calls Yuki, he's both ashamed and pleased at how Yuki feels towards Haru at the moment, even if it's not fair of him to feel that way. Haru doesn't deserve it, Natsuki knows. Friends aren't supposed to doubt each other, not like that.
So when he shouts, "He's your friend!" into the phone, hew knows something in him isn't saying it to convince Yuki but rather to feel reassured - Yuki and Haru would never work. It'd never work, is what he wants to feel, and Yuki saying that he doesn't understand Haru is both the best and the worst feeling Natsuki has had.
I'm not playing fair. Natsuki grits his teeth, his sense of justice wounded. Acting like this...What kind of friend am I to Haru?
But then he thinks of Yuki, and his bright, bright smile and shy words and soft lips, and Natsuki is, well, defeated. When he thinks of all of that being stolen away from him, snatched so easily like bait from a hook, he's furious. To have Yuki, to be able to hold him, he is the only one. He has to be the only one allowed that privilege, and everything be damned if anyone else ever had it.
Still, guilt trips him. He gathers himself and decides to help Haru, but even that has a second motive in the back of his mind, a dark mutter of "Then he'll leave Enoshima", because then nobody can easily take Yuki away from him.
That is the only word in his mind as he sets towards Enoshima, prepared for anything.
de-anoning from the kink meme [[I wrote this at like 11 at night oh god]]