I thought the book implied that Regan's possession came about because she hated the guy her mother was involved with (with good reason, he was absolutely revolting) and then the demon killed him and she felt guilty about it. But they also implied that the original contact with the demon was through the Ouija board games with her imaginary friend.

The scary parts of Regan's possession were definitely disturbing, but for me none of the rest of it worked. The dialogue was downright ridiculous and so were several of the characters, particularly the policeman.