I don’t entirely agree. If you can help, you should. Not the to the point of suicidal martyrdom, but a small shared risk is better than making a single person alone bear much more.
The reason you put your own mask on in an airplane first, is that without it you lose consciousness very quickly.
The low oxygen environment won’t instantly kill, either, so you’re not actually trading in their chances for yours. After you get your own mask on, the person next to you will be perfectly fine even if unconscious, as you can now get their mask on in their stead.
It’s simply the most logical thing to do, all factors considered.
There are plenty of emotional reasons for people to take a bullet for someone else.
But even beyond that, there are cases where a bit of self-sacrifice makes logical sense, and is the right thing to do. Prioritising your own chances even over a win-win scenario, would be kind of a dick move.