I think it's because Laura's dead that McPherson allows himself to fall in love. He knows from her friends and surroundings that she was intelligent, creative, dynamic, caring, and beautiful, all qualities that would have drawn him to her in life. But being already dead Laura is safe. She'll never be able to deceive or disappoint him, never be common or cruel or false. I don't think McPherson had good luck with "dames" in the past. I always hope at the end of the movie that he was so far gone on Laura that he can get past his idealized vision of her and fall in love with the real woman.