In my head, seeing as Steve probably has next to no experience in being with another person, he’s sort of awkward about it and doesn’t know how to go about showing affection. Especially when another man is the other person. Most importantly, a Stark. Intimidating ahoy.
So Steve always finds little, other ways to show he’s paying attention. Things like pulling lightly on Howard’s suspenders when one of them is wandering by and no one’s looking. It might not be something like a legitimate touch or anything of the sort, but it’s enough to express what he means.