What does Snape owe Harry?

[slytherin_jenn]

No: HPFGUIDX 96702

Ok slightly off topic but I've wondered about this as soon as I 
discovered the circumstances of the life debt Snape supposedly owes 
James. James stopped Severus from being killed in a prank that his 
best friend orchestrated; and was saving Sirius as much as he was 
saving Snape. If Snape was killed than Sirius would be in serious 
trouble, Lupin would be exposed as a werewolf and Dumbledore's 
decision to allow a werewolf to attend Hogwarts could earn him some 
reprecussions as well. In other words, he had a lot of personal 
reasons that had nothing to do with Snape for doing what he did.

So why exactly does Snape owe him a life debt? I don't see the act as 
being particularly magnanimous on James Potter's part and considering 
his conduct toward Snape I would not think that Snape owed him 
anything, much less his son and yet Snape has gone out of his way to 
keep Harry from getting killed. Unless of course there was more to 
the story than we know so far which I suspect there is. 

Is it any instance where someone saves someone's life whether on 
purpose or not. By the same logic one could say that Harry owes 
Lucius Malfoy a life debt since he didn't allow Bellatrix to kill him 
while he was holding the prophecy. I wonder also if there are varying 
degrees of life debts? For example if someone really goes out of 
their way to save someone would that life debt be greater than 
someone who accidentally saved someone's life? 
Sorry for the shameless Snape support :P
jenn