Kakashi can't cook.
Oh, he can survive on his culinary skills, but onigri and instant ramen should in no way be the basis of one's diet, let alone all that one eats. So Iruka always cooks, because he knows Kakashi plus a stove equals not good things.
But that doesn't mean he won't gripe about how he feels too much like a damn housewife and how it's just plain not fair and why can't Kakashi put some effort into learning? But he cooks anyway, because he know that even though Kakshi says he's not that hungry and Iruka shouldn't worry about it the man hasn't eaten since lunch and he's had to suffer through working in close proximity to three teenagers and god he's surprised Kakashi doesn't have a continual headache.
So Iruka cooks and Kakashi leans on the counter and they talk over the sizzeling hiss of food in a hot skillet and once everything's done Kakashi kisses his cheek without even removing the mask and then promises to make it up to him. So maybe cook isn't the thing Iruka despises most in the world, but he's still going to hit Kakashi when he discovers his birthday present is actually a frilly apron and he's never going to stop complaining about how a grown man should have the ability to cook one half-way decent meal. And maybe it's gotten to the point that the grumbling is more just habit, but Kakashi doesn't need to know that. And he doesn't need to know that little kiss Iruka receives makes it all worthwhile.
Still, it's doubtful he'll ever admit that all the guilt he's been causing Kakshi is unjustified. What would be the fun in that?
Iruka in a frilly apron. Drool. God that's a hot image.