Forum:Dumbledore and Snape

I thought about this for some time and I still don't get it. Why did Dumbledore trusted Snape? I know that he's been right in doing so, but I can't understand what could make Dumbledore so sure in his trust for Snape. Was it because of Snape's love for Lily Potter? If it was, then how could Dumbledore believe that his love was genuine, knowing that Snape was a highly accomplished Legilimens, and capable of disguising/hiding his lies?