well, steve’s defining characteristic is always that he will do the right thing, no matter what it costs him personally. only this time, i think - and you’re right - he has less to live for, outside of the work. all his friends are dead, peggy carter will probably die very very soon, and he’s increasingly disillusioned with the organization he works for; not to mention - and this is probably one of my favourite parts that i somehow forgot to put in the review - he’s obviously suffering from some kind of survivor’s guilt and ptsd. that’s why, i think, natasha tries so hard to find him a date; it’s not about settling down, it’s not about having a good time or replacing peggy carter - she’s trying to find him an anchor. but the fundamental difference is that though natasha can reinvent herself over and over to survive, that she has the flexibility and willpower to look forward and not back, to keep moving and atone instead of staying back and risk losing herself - steve can’t do that? which is why the framing of this film, as him literally confronting the ghosts of his past - hydra, bucky - is a kind of healing process that works for him the way sam’s therapy sessions work for the others.
so in a sense, i don’t think he loses the will to survive once bucky shows up, i think he just - like his first face off with hydra - accepts the fact that he may have to die, in order to make things right. its interesting, because if you look at these two passages of dialogue:
SAM: what makes you happy?
STEVE: i don’t know.
NATASHA: you seem pretty chipper for a guy who found out he just died for nothing.
STEVE: i guess i just like to know who i’m fighting.
he needs a purpose. he’s not… happy, per se, by the end of this conversation with natasha, but he has a purpose. he knows what he has to do, he knows what is necessary, and so long as he can fulfil that purpose he is more or less, a functional human being. so when you put him on that falling hellcarrier with bucky; he’s not happy. he’s the furtherest thing from happy - but he has a purpose. he had a purpose when he made the decision to crash red skull’s plane, and he has a purpose here. between his life and stopping the bomb from hitting new york, between his life and bucky remembering - well. it’s a small price to pay.
and so, i don’t think he went out on to that plane looking to die. you’re right - he would probably have made more of an effort to survive if bucky had been alive still, but remember that peggy carter was alive and waiting on the other end of the radio, and that didn’t change his decision. certainly bucky’s death is what galvanizes his decision to go after hydra - compare ‘i don’t want to kill anyone. i just don’t like bullies’ with ‘i’m not going to stop until all of hydra is dead or captured.’ - but if bucky was alive, and the decision still came down to between him and the millions of people who would die?
the decision would still be the same.
with regards to bucky, i think the underlying idea here is that even from the very beginning, steve didn’t want to be useless. he didn’t want to be a liability on anyone, and he didn’t sign up for the war for glory or for vengeance (considering this was post pearl harbour) or because he wanted to kill nazis. he did it, very literally, because he felt a duty. because ‘there are men laying down their lives; i got no right to do any less than that.’ so the thought behind this, behind crashing the plane is - this is what happened to bucky. bucky died serving his country, crashing into the alps. he will have an empty casket because they never found a body - he will never have a decent paying job, he will never have a wife or children or grandchildren, he will never get to grow old with them and he will never get to come home from the war with steve. all he had and all he will ever have is that icy death in the alps; because he served his country. he laid down his life.
so steve’s not looking for death. when push comes to shove and a decision needs to be made - steve, well. he’s got no right to do any less than bucky.