One thing I don't get about Joy

She's such a sweet and kindhearted girl, so why is it that in both this film and its prequal, she falls for a sad, pathetic, emotionally unstable, and to top it all, suicidal man? Surely she can do better. I find it hard to believe that someone as straight and honest as her would marry someone like Alan, who was into drugs, crime, gangs and (presumably) obscene phonecalls.

Also, why would she fantasize about her dead ex, Andy, but reject his advances? And why fantasize about him turning nasty, even trying to rape her?