It did not make sense that he would fall for her. She had severe mental issues and was a very negative person who argued with children even over riding sea turtles. Then as the plot progressed she got normal and accepted she wasn't wanted and went away. It wasn't consistent with her character at all, and they tried to make the real Missy look bland when she was a normal, engaging, well adjusted person.
So the guy falls for a lunatic, and the lunatic just goes away like that when she was extremely crazy the majority of the time.