In my imagining, there were many people lost in the war. Sirius and Snape, especially, have no one left to relate to. They could start over, but no one would be able to understand them better than...Remus and each other. This is their only chance at a real life and if they fail - it's mere existence city. ;)
Thanks for being so supportive of the series. I have another installment percolating in my brain now. *g*