In light of Severus Snape's atrocious treatment of Harry Potter, not to mention the fact that he killed Albus Dumbledore, many fans wonder why Dumbledore trusted him and why Snape protected Harry, a boy he clearly hates, for the past six years.

I can't remember where I read this theory, but someone surmised that Snape may have made an Unbreakable Vow to protect Harry, which is why Dumbledore was certain of his loyalty and the reason he protected the boy wizard.

We know that in Book 6, Snape made an Unbreakable Vow with dark witch Narcissa Malfoy. Apparently, Snape vowed to kill Dumbledore and protect her teenaged son Draco if he were unable to do it. Because of the vow, he was forced to kill a man he obviously cared about and protect a rotten kid like Draco. The same could be said of Snape's protection of young Harry. I haven't fleshed this out yet; I just wanted to get these thoughts down.

Also floating around is the idea that Snape made a "general" Unbreakable Vow of loyalty to Dumbledore, but not specifically to protect Harry. Interesting…