In which Thor takes pleasure in something he really shouldn't.
Thor hesitates to admit it, but in his way, he actually enjoys it when Loki is ill. It's not that he likes to see Loki hurting - that couldn't be farther from the truth. There's not much he wouldn't do to relieve his brother's suffering, were it in his power. It is simply that only during illness is Loki open to the affection Thor so longs to give him. It might be a remnant of their childhood, that Thor so likes to take care of Loki, but if it is, it is one that Loki usually does not share.
His brother, though he may be slighter and softer than Thor, is fiercely independent. If Thor were to offer to dote on him as he wishes, Loki would certainly take it as a slight. And so he holds back - Loki has already more than proven that he is capable of taking care of himself. (If only he could see that he had never needed to prove that to Thor, but that would require Loki to be someone other than who he is, and Thor has never wanted that.)
But when he is ill, Loki is… well, for lack of a better word, he is whiny. Needy. Nothing like the brother Thor otherwise knows. Thor takes these opportunities to get his fill of caring for Loki in the way he is so often denied by Loki's own pride. It is not like there are others flocking to the task, after all.
Even their own mother finds Loki's complaints trying. He is especially bad with fevers, complaining of being too hot the one moment and too cold the next, requiring that blankets be added and then removed only minutes later. After a day or so, it is only Thor who can stand to be around him anymore. Of course their mother comes to check on him at intervals, but Thor sits at his bedside day and night, tells him stories of their childhood and cards his fingers through his hair, allows Loki to blame him unfairly for all sorts of ailments. Loki looking so miserable makes Thor's heart crumple in affection - his brother is usually so fastidious in his appearance - and for once, Loki allows Thor's protective instincts to flair, pink spots of fever on his high cheekbones.
Loki is sweaty and aching and restless, yet Thor delights to have his brother in his arms, rubbing his back against the shivers that wrack him. Guiltily, he smiles a little into Loki's ear, arranging them so his brother will be as comfortable as possible, Thor acting as his pillow. Surely Loki would never allow anyone to see them like this, but the door is locked, and he is too sick to care. They both know Thor has always been something of a security blanket to Loki; it is millennia too late to deny that.
Thor cradles him close and plies him with juice; though Loki complains that his stomach hurts too much for anything at all, he eventually acquiesces after a little pleading (Thor always knew he would anyway), and although he refuses to admit it, it's clear that he feels better for it, relaxing in Thor's hold and allowing the blanket to be wrapped around them again.
There are many things Thor is good at - he is a fearsome warrior and his hunting is hardly rivaled throughout all of the Nine Realms - but there is hardly anything he loves so well as coddling his little brother. Their mother says Loki has Thor wrapped around his little finger, and perhaps she is right. But as Loki snuffles in his sleep, sounding almost grossly congested, and burrows his reddened nose farther into Thor's shoulder, Thor just holds on more securely.
Perhaps it is wrong, but he cannot bring himself to mind.