Why did Obi-Wan Kanobi let Darth Vader kill him?

I always thought, ever since I was young, that Obi-Wan did it purposefully to motivate and drive Luke. if you see, Obi-Wan briefly glances at Luke while he and Vader are fighting, and he smiles. I always took this as Obi-Wan's way of spurring Luke into devoting himself to the Force and defeating Vader, knowing that Luke would be furious after seeing him cut down by Vader. Just my opinion, though.