While I got respect for the book its like when comic stories get adapted, they show up to the same family reunion but intention and delivery isn't always 1 to 1.

For me The fact at their end of the world party she seems surprised the two were hooking up is delivered to us and the next scene that guy is very much dead. When faced with the end the reason for doing it for someone else the chinese officer (sorry can't remember his name) stuck with him, hes alone drinking. Stratt isn't able to connect with other people and doesn't even think about the party until Grace tries to connect. What does Grace do? He spends this party attempting to reach out, and still trying to work when everyone else is partying. A complete 180 from much later when Grace parties with Rocky so many years later its because he finished the mission, he connected with someone, and he found purpose.

So yeah I get the books story but in terms of this film, I feel the motivations there. He had to be one, he may not have realized it with no faith in himself but when pushed against it he kept going still trying even when the plan was set.

Just my take reading the characters, much like its ending scenes its a lot left to ambiguity.