Steve gets de-serumed and that makes him super grumpy so extreme sassy!Steve comes out. He finds it both annoying and endearing to know that his boyfriend laughs at all his sassy one-liners even when they are directed at said boyfriend.
Tony loves his boyfriend even when he is a small ball of ‘fight-me’ and sass. Tony thinks that he might like Steve a little better this way (not that Tony doesn’t like or miss his boyfriend’s hot body but somethings are just more fun when they are seen in a different light).
Steve gives Tony half-hearted glares and looks of annoyance but inside it makes him super happy to know that Tony loves him for him and that he loves his sass. Rarely anyone does. Most just tolerate it but Tony...Tony loves it and where there is Steve (especially de-serumed! Steve) there will always be the sass along with him.
Tony would not have it any other way.