You do get to just shrug it off. You are not responsible for anybody's happiness. If you are unhappy, move on. The fact that a person's is making the choice to move on and not drag out a situation that will spiral into miserable existence means they made a good choice. Maybe you're the one who "must be very young" and by young, I mean lacking in experience. Because after a point, your age number correlates much less with life experience.