Why Was Sam Spared?

We’ve already discussed the White Walkers’ ruthless ways, but there is one particular moment that goes against their usual way of operating. In the Season 2 finale, ‘Valar Morghulis,’ Sam comes face-to-face with one of the White Walkers’ leaders. The White Walker stares directly at Sam and does nothing. Towards the end of Season 3, another White Walker has the opportunity to kill Sam. In both instances, they decide against it. Why?
Image Source: IMDb
