The Story of Silverstream's Parents

Silverstream is a warrior of RiverClan. Her mother is Willowbreeze and her father is Crookedstar. She fell in love with a ThunderClan warrior, Graystripe.

I heard it say that Silverstream and Graystripe shouldn't have met. But why not? I mean the pretty slender silver tabby she-cat saved Graystripe from drowning, so it's not that bad, right?