"Why do you think I'd be happy with you altering your genetic code, screwing around with your DNA, and tempting fate to do something horrible to you? I don't care how much you've perfected it; this soldier serum has fucked up peoples' lives everywhere you look, and I wouldn't be surprised if this one killed you or worse."

Because Bruce knows sometimes dying is the better way to go. That Tony would think Bruce would want a symbol of devotion like this is actually enough to get him angry, and he pushes himself up from the couch, needing to distance himself, needing to gather his faculties and let himself be angry without being too angry, or they'll run into some problems.

"You really think this is something I'd want? Yes, of course I love you, and I'd like to be with you as long as that's possible, but Jesus, Tony. Can't you see how this is like me standing by to watch one of the worst things that ever happened to me, go and happen to you?"