Which one hogs the blankets?
Which one cuts the other’s hair?
Which one makes coffee for the other one every morning?
Which one picks up the pizza?
Which one likes their music on full volume?
Which one complains about the crumbs on the bed?
Which one is ticklish?
Which one sings and which one plays the music?
Which one proposes?
Severus hogs the blankets only because Harry gives them up too easily, having learned to do without as a child. Severus doesn't mean to steal them, but Harry pushes the blanket around enough that Severus ends up with them.
Severus tries to tend to Harry's hair, but it has a mind of its own. Severus won't let Harry near his hair for anything.
It's actually tea, but Harry makes tea for Severus. Both are early risers, something beaten into both of them by life and necessity. At first, Harry and Severus stumbled over each other, both trying to be helpful. After some careful negotiation, Harry took over responsibility for breakfast and Severus took responsibility for dinner.
Harry brings home pizza, to Severus' secret delight, although Severus will grumble about "pizza, again?" when Harry does it. Neither really had much pizza before -- the Dursleys would never have such a 'common' meal, and Tobias never wanted to spend the money on it. So pizza was a foreign food to both Harry and Severus. Harry really likes Thai food better, but he knows how much Severus likes pizza, so he tries to bring it home pretty regularly.
Neither likes music at full volume. Loud music can cover the sound of someone trying to get near them, and years of heightened awareness has not gone away. Letting someone too close and being caught unaware can lead to a dangerous situation.
They both complain, but Severus is very aware of Harry's miserable childhood and encourages Harry to eat when and where he likes. And if there are crumbs in the bed, then Harry's comfortable enough to be eating more. So while Severus will complain, it's more to maintain the illusion of grumpiness.
Harry is ticklish. Severus trained himself to not be ticklish at an early age, because his father would otherwise tickle him until Severus was breathless and crying. Tobias would say it's "all in fun" but then say "only sissy boys cry" and harass Severus until Severus learned to not react. No one ever touched Harry to learn if he was ticklish or not. Harry and Severus realize this in bed one night when Harry reacts to Severus' touch. Severus is conflicted, since he wants to be able to tease Harry but has such a negative memory attached to tickling that he's afraid to do it to Harry.
Severus plays the music, although he never really learned to play an instrument. Severus is enamored by a record player Harry found in a second hand store and has built an eclectic collection of records, mostly classical. Harry tries to sing but is tone deaf.
Both Harry and Severus suffer from "he's too good for me" and both worry that the other will "find someone better" and neither wants to upset their relationship by proposing and getting rejected. It isn't until Neville's wedding that Severus realizes he wants to spend the rest of his life with Harry and he should take the risk. Severus corners Harry in a quiet alcove and is amazed when Harry says an unequivocal "yes!"