When it comes to parent's and children the parent has a responsibility to raise their children.
It's part of the agreement they signed up to when they had a child and chose not to abort or put the child up for adoption.
Also in your example it's not personal selfishness the other parents shows, it's care and concern for a child's well being.

But even in this case though, if the parent is happier leaving the children then let the parent leave.
Studies have shown that children develop worse when parents are together out of a sense of necessity rather than having separated parents.
Plus even without those studies it's fair to argue that a parent with no desire to be with their kids is probably one shitty parent, hell there are tons of parents who do want to be with their kids who are god awful at parenting.