It's not being difficult when you have mild offense at offering a complete stranger a free pass into your home while carrying something that could easily transfer into a weapon. Tony has made guns, many, many guns in his life, and he's had some of them aimed back at him, so he's rightfully paranoid.

Tony utterly ignores that look from Steve and jumps inside to tidy up the place a little, hopefully before Isaac can see them. Like some very particular design specs on some transfer paper, equations for very specific things like arc reactors and armors, and various tidbits of metal and wiring littering the couch.