(Ron) Dumbledore asked him to. Remember when I helped him find that locket, and he had to drink that poison? That coupled with the ring he destroyed, it was too much. He knew he only had a year left at the most. He asked Snape to kill him, so it would look like he was still loyal to Voldemort. Because of that, we were able to do so much more.
I know it sounds mad, Ron, but I need you to trust me. I saw it all myself, in the pensieve in Dumbledore's office.