Oath
Blood oaths were not common among pureblood bloodlines.
They were strong; physically and magically binding.
These oaths were only created in acts of desperation -- in life or death situations, and death did not seem like a path one would want to follow.
So when Voldemort had come to him, offering up a truce – a pure blood oath – he had accepted. It was a means to an end, at least at that moment, and he needed that if nothing else.
Harry was foolish to believe that the Dark Lord would keep his word, even through binding magic. Harry realized that now he was years too late in saving the world, and years too young to make any difference.