Well they had that in common, didn't they? Severus's only stable -- and that was up for debate -- connection with the outside world was steeped in the 'things' that Remus mentioned happening outside of those walls.

Severus was seduced by the owls he received about them...though since he'd met Remus, he had been thinking more of the things he could accomplish with the other boy.

Animagi spells, a patronus! Remus was his muse and lucky charm.

"The world is dangerous already. Fact of the times. You're in the classes you need to be an auror. Dumbledore will get you in."