Jackson is a wolf in Derek's pack, and he is jealous because he thinks Scott gets more attention and respect from Derek than he does. This jealousy may be based on fact or it may not. Either way it is slowly making him lose his mind. And he does his insecure overachiever thing - anything he thinks might attract Derek's approval, he does more and better and harder than anyone else. He works out more, he studies more, he beats up wolves from other packs more - during the full moon he runs faster through the woods and kills more wildlife than anyone else. But Derek just seems oblivious. Frustration eats away at Jackson.
He hasn't gotten a good night's sleep in ages. One night, Jackson is running through the woods, exhaustion catches up to him, and he passes out. Derek is the one who finds him. Jackson is all naked and chiselled and bloody and unconscious. Derek just sort of... looks.
Jackson opens his eyes and catches Derek looking.
Neither one of them has ever considered the other as a sexual partner before. But things shift. Derek looks at Jackson when he thinks no one will notice. Jackson can't believe this was all it took to get Derek's attention. He waits for something to happen.
Nothing happens. Except for staring. Derek watches Jackson... but hardly ever talks to him. He hardly ever speaks, it seems, unless it is to speak to Scott. He even asks Scott's advice. Jackson is annoyed. Aggravated. Vexed, even. Weeks go by, with Derek watching Jackson through his eyelashes, Jackson expecting to be cornered in the locker room or invited to the house in the woods. He loses patience, grows peevish. He digs out the tightest pants he owns and starts removing his shirt at the slightest provocation. But Derek either has unnaturally powerful self-restraint, or turning into a wolf has, perversely, deprived Jackson of his ability to recognize attraction. Nothing happens. Derek's pupils seem dilated sometimes, but he seems to go out of his way to make sure he and Jackson are never alone in a room together.
Jackson snaps. One night - after Derek spends an entire evening watching Jackson's new obscenely tight pants (he wore them sans undergarments) only to turn away at the end of the night - Jackson goes to the house in the woods. He suspects it might be bad form for him to show up unsummoned at an alpha's door, but all his thought processes are fuzzy, he can't think, he can't - he finds himself walking straight in and climbing on top of Derek, in his bed. Jackson has no idea what is happening. He's not sure what he wants; he smushes his face in Derek's neck. He didn't intend to do it, he intended to demand an explanation of Derek's puzzling behaviour. It just happens. Jackson nuzzles Derek and smells him and feels both frenzied and oddly relieved at the same time. Derek wakes up and groans.
“Jackson”, Derek says, with a let’s-be-reasonable voice, “Jackson what are you doing”.
And Jackson snaps, he pushes Derek down into the mattress, holds him down, and Derek’s pupils become so big so fast, it looks like it must have hurt. Jackson grabs his hair (he has no idea what he's doing) and grinds down onto Derek’s thigh, and Derek makes a sound and goes all limp, and it drives Jackson insane because why, why has Derek been ignoring this, why has he been... putting it off, why has he been all passive and weird and trying to pretend he wasn’t staring at Jackson all this time and now he’s sighing and making keening noises like his whole stupid Derek self isn’t made up of mixed messages and confusion.
Derek looks dumbfounded, and just as surprised as Jackson when they start really going at each other. When Jackson paws at his t-shirt and rips it and pulls it off, Derek appears to be - could he be - he's been blindsided by lust. And he - his legs fall open but he simultaneously asks “wait wait what?” and that enrages Jackson. "Shut up", he tells Derek, as they roll around in the sleep-warm sheets, "no waiting. You don't decide anymore. I decide." And Jackson rips Derek’s boxers off and holds him down and pushes into him.