Note 9: Falling in love with your best friend is acceptable.
I think those have to be the best type of stories. A boy meets the girl next door and they fall in love after the boy realizes that the hot popular girl is not his type. Or even a girl watches her nerdy guy friend turn into a hot hunk of man over the course of one summer. Those are just fine.
Note 10: Falling in love with your best friend who also happens to love every single girl he's ever seen is NOT ok.
See, I can't even think of a plot that encompasses those two things: love and whore-ism. Two best friends fall in love. For a day. But then the boy best friend sees a hotter and much prettier girl and then leaves with her, but then breaks up with the hotter girl and leaves with an even HOTTER girl. The girl best friend becomes horribly, horribly depressed. She gains a lot of weight. Then she starts sleeping around with a bunch of gross older men. Then she gets pregnant. And she can't even move out because she works for the best friend. She turns fat and ugly and he doesn't even want to drink from her but he has to and thus begins to resent her. Her friends and family begin to hate her also because she's bitter and unlovable and fat. Her belly becomes so large that is actually swallows up the baby and she becomes eternally pregnant. The boy then finds a woman who is not gross or fat or forever pregnant. He marries that perfect woman and they have kids. The girl is then moved up to the attic because she is now the shame of the Hunter family. The boy sends servants to bleed the girl because he doesn't even want to look at her. The girl is all alone. She has to watch the only man she's ever loved have his happily ever after for the REST OF ETERNITY. You know why?
Because they're both immortal.