Dan is gone.
Ever since he got in that car accident, Phil hasn't been the same. Yeah, sure, he was in the hospital for a few months, but that didn't matter because he still died. And Phil was mad. He's mad because Dan, his best friend, is dead. Completely, utterly dead.
Is heaven even real? And, if so, did Dan go to heaven? Or, did he go to hell, if there even is one. How can someone know?
At the end of the day, Phil just misses Dan. He wants to be with him again. So, he tries to find him.
But, he's stuck on earth. He's dead, but he's stuck on earth. He just wants to find Dan. And that is his mission.