It’s part of Irvine’s philosophy in life and junk. There’s a lot (a few) of points where he talks about choice and he sort of is the only character who really gets to truly consider his choices as a mercenary since he doesn’t suffer from memory issues.
He makes a speech at one point (can’t remember if it’s this scene) about how they’ve always had choices, but sometimes you really don’t. Sometimes life only gives you a single choice that’s actually viable, but you still have to make the choice either way. A major example being how he (and the other orphans) had the choice to become mercenaries, but really they didn’t have much of a choice since their lives where sort of predestined down that path due to choices made for them by the previous generation. But even with this they still arrive at where they are due to the choices they themselves have made. Cause you can have an infinite amount of choices, but sometimes there are options you simply have to take.
It sort of ties into the games bigger themes of destiny and free will and the meaning of choice (the time travel stuff ties heavily into this). Because even though they were raised to be soldiers it’s a big point of the Trabia Garden scene that they all actively choose to stay soldiers. It’s at that point that they stop fighting because they are supposed to and start fighting because they choose too.