Tony didn't feel all that great for getting a laugh out of Steve because it wasn't a very good laugh. Mostly it was just sad and it made Tony sad and it was just a mess of feelings, really. He felt --
He didn't know what he felt. Sorry, mostly. Because it didn't seem fair to anyone how Thanos had gone down; how one person had been cosmically allowed to fuck up that much.
And it didn't seem fair that these were memories Steve hadn't even had to live through yet, but here he was trying to sort them all out like he had. It was an unjust punishment that came in ways that Tony couldn't comprehend in order to fix. "Hey," he said quietly when Steve closed his eyes, because he looked a bit like he was going to be ill. So he squeezed Steve's hand a little harder.
"I'm sorry," he said, and he found that he meant it. Because that was the problem, really. Tony's sacrifice in the future was probably 2 parts selfless and 1 part selfish. The dead didn't have to be around to see how people mourned or couldn't go on, after all. Some other version of Tony never had to see how it affected Steve, Pepper, Morgan or anyone else. "I'm sorry," he repeated. "For leaving you before we even --" he paused, unsure.
They had here and they could repeat that like a broken record, but that didn't cover everything and they both knew it. Steve now more than ever.