^Steve's role/response in that reminds me of some of the WWII veterans my grandparents knew (go figure). They'd already been though one war and in spite of all the hardship they went though, thought the outcome was worth it.
I think sometimes in the MCU it's hard to remember that Captain America has already fought in a long continuous war while the other Avengers really haven't. They're starting to realize that the current (alien) conflict is shaping up into one and not all of them are sure that they can handle what that means for them and the world. For all that Captain America has trouble fitting in with modern society/culture, he's been though a large-scale war already; it would be stupid for the other Avengers to write off what he thinks even if it happened a long time ago for them.
It's also interesting contrasting Steve's vs Tony's childhoods. Tony has always been good at what he does; the type of frustrations he experiences are the frustrations that come with knowing you can do something if you can only figure out how to do it. And then later he has the frustrations of watching his creations blow up in his face because other people didn't use them correctly (in his opinion). His solution to all that is to take the creation that failed out of the equation all together. I kinda get the feeling from the trailers that that's how he's viewing the Avengers now; the Avenger experiment failed and now it's time to try something different.
Steve meanwhile was never good at what he wanted to be. And what he wanted to be was someone who could defend people weaker then himself. What he has gotten good at is empathizing with people so he can know how to defend them better. His frustrations come from not being able to do what he knows will help fix a problem. He never looses the motivation to help people once he's able to do something about it either. From what we've seen, I think he's seeing the Avengers as people who need protection from the very people they're helping keep alive. Whether or not he's capable of doing that isn't something that Steve cares about (so long as he's actually trying) and I don't think Tony gets that.
It also looks like Steve's realizing that none of the Avengers will be able to go back to "normal life". It's been reiterated in several trailers that Bucky isn't involving himself in the typical superhero behavior (as far as the world is concerned, he disappeared pre-Ultron mess), but the government is still going after him anyway. It doesn't take a huge leap of logic to think that the government would go after the rest of the Avengers once they're "done" with their superhero lives, if they didn't like what they'd done while they were superheros. Which would make Tony's question of "Aren't we we fighting to end the fighting and go home?" kinda moot. What's the point in trying to end the fighting if people are going to come after you regardless of if you're fighting now or not?
The other thing Tony is glossing over is that the Avengers aren't the ones who started or are continuing the extra-terrestrial/dimensional conflicts (Loki and ultimately Thanos did). The aliens are coming no matter what the Avengers do and it sounds like Tony's not thinking about what would/could happen if the Avenger's hadn't been there in the first place...
