At the end of *Philosopher's Stone*, Dumbledore tells Harry that Snape was trying so hard to protect Harry because James once saved Snape's life.

He could have said the complete truth, something like:

> Snape hates you because you look like your father, who was a bully
> who, together with Black, made Snape's life impossible and once almost killed him. However, Snape will do anything
> to protect you because he loved your mother.

Or, if that was supposed to be a secret, he could have said a white lie, only telling part of the truth:

> Snape and your father didn't get along very well. But there is no way he
> would let any student die, if he can avoid it.

But why did he tell that lie?