Steve slowly turns to him after he asks the question. He wants to be clear on this if nothing else. "I couldn't have fought with you like that in New York if I didn't trust you. I knew you'd watch our six."
He works his jaw as he tries to figure out how to deal with the rest of it. Despite how hard he's fighting against the reality of the situation, he does respect Tony enough to try. "I know you're not a kid. You're older and more experienced than I am when it comes to being... I don't know. A living person? I don't discount that." As much as he is willing to play along with the old jokes, he's not. He's a twenty-something who is hurt and utterly lost. He's likely the youngest of all of them when he allows himself to think about it. If he's older in any way it's only because he voluntarily threw away his innocence to fight and kill like the rest of the kids from his generation.
"But, I don't want you to suffer. I don't want you to be me. Do you understand? I spent years watching people die almost every single day. I could only lessen some of that suffering and death then. I could only die when it came down to it. That hasn't changed."
He pauses only briefly before pressing on with the rest of his speech. "You spend your days making sure that never happens. You find a better way every single time. Why would I ever want you in the mud with me trying to solve my old problems? You're better than this. I think we both know that." There has to be a reason why his friends always live while his die. That's the only explanation he can come up with. Tony is better so why throw him into his hopeless situation? He can't imagine a reality where maybe a partnership would bring about the hope that has been stolen from him. Despite himself he really does treat Tony like a child to be shielded from horrors whenever possible.