Prompt maybe? Or just thoughts/headcanon needing an out -- Magnus thinks of himself too much, still fears that he is actually still too much for Alec, but Alec fears that Magnus will find out, with a sort of heartbreaking turnaround, that Magnus unknowingly instilled that fear into Alec. That Alec himself thinks he is too much, takes for granted, asks and asks from Magnus, that he can be and is at times, too much for Magnus.
They’re so perfectly balanced sometimes, aren’t they? Magnus’ particular style of self-defense, the so cautious parting of layers and layers of caution and deflection, hits a perfect bulls-eye against Alec’s overdeveloped sense of personal responsibility. Because any time Magnus metaphorically flinches from confrontation, from conversation, from their relationship, it has to be Alec’s fault, right? (Everything’s always Alec’s fault, if you ask Alec.)
Magnus has centuries of history that taught him over and over again that no one else could be trusted to protect him and that he had to do it all himself. But it hurts Alec each and every time it doesn’t occur to Magnus that he could let Alec in rather than protecting himself, by himself, for himself.
On Magnus’ end it’s never because he doesn’t trust Alec, but because he doesn’t want to burden him, because he’s just used to there not being someone there in the wings waiting to help brace him up before he steps out on stage again.
But Alec’s entire self-worth is based on the fact that he does brace up people, that he’s a better support in the shadows than a performer in the spotlight; even when he’s in the spotlight he’s cautious about it, steady and studied, never flamboyant, never a show. So every time Magnus pulls himself together on his own, it is, to Alec, on a visceral emotional (entirely illogical he knows this but he can’t help it) level an overt rejection of everything Alec feels he can offer.
Alec’s issues with Magnus’ immortality have nothing to do with being jealous of Magnus’ past or future, but on this further evidence that, to this person who means more to him than anyone else in the world, he is fundamentally useless.
And Magnus is so used to other people judging him on his own use, his power, his skills, his knowledge, his history, that it never occurs to him that Alec desperately needs the opposite of Magnus’ dramatic and performative competence. Especially since he does let Alec take care of him in a way he’s never let anyone do before; it’s just it’s in ways that Alec doesn’t recognize, because Alec does not value his own sincerity and stalwart appreciation of those he loves as anything beyond basic humanity.
Magnus is in love with a man who sees him, not his magic or his heritage, and is delighted to be able to offer his skills because he wants to, not because someone else expects it of him.
And every single time he does Alec’s heart breaks because he cannot offer the same sort of protection back to Magnus.