1)James was bitten the night he saved Snape from Remus in the shack though noone knows, the marauders stopped spending the moon with Remy. Keeping it a secret from everyone but Lily and Remus, James goes on to father Harry Potter, the first pure blood Lycan in centuries. Harry being born a werewolf can completely control the change, though he does spend the moon with Remus, who Harry can sway even without the potion. severus learns to love Harry despite the Lycanthropy.
2)Harry absorbed Voldemort's agic when he killed him, now having the magic of three very strong sorcorers(Lily, Voldemort and his own) Harry has gained immortality, his magic keeps him alive. He somehow- up to you- saves Snape and shares this gift with him through a bonding. Love comes but it takes a awhile.
3) Harry is a thief and severus and his team of wise cracking smart arse Aurors are after him.