How does a wolf imprint on something?

In the Twilight novels, imprinting is a sudden and uncontrollable love for someone. Kind of like love at first sight, and it is a two way street.

The love is pure, and not just a sexual lust. For example, it is possible (but rare) for a werewolf to Imprint on an infant. He will be a protector, friend, and sibling to that person until they are old enough to have a more adult relationship.