There also seems to be an overarching theme that you have to accept pain to draw hope from it.
If Shart frees herself from the wound and Shar's influence, she looses her parents and her chance to discover more about herself. The guy you meet in front of the House of Grief surrendered all his memories to the mirror in order to rid himself of the pain the loss of his wife (?) caused him, but in doing so, he also forgot about all the good times they shared. By contrast, if you allow Gale to sacrifice himself at the end of Act 3, he will try to cheer you up by telling you that all the happy memories you made together will live on in you. All the good endings, too, are more hopeful endings really in which the characters distance themselves from a firm belief they previously held to gain the chance at a better, more fulfilling life. Even Ketheric is a much easier opponent once he is reminded of his beloved wife.
Selune and Shar both deal with extremes and have little understanding for the things in the middle, it's either light or dark but Shart's realisation that you need a little light to cast a shadow is lost on both of them. Even Dame Aylin is fairly ill equipped for this more complex understanding of the world, as seen by her reaction to defeating Lorroakan.
