No-one knows why but cats fall in love as humans do but I would say because Whitewing is a young and pretty she-cat and in the end they have kits
Whitewing
Ivypool's parents are Whitewing and Birchfall. Whitewing is her mother and Birchfall is her father.
At the end of the third series whitewing has birchfall's kits.
Ivypaw's parents are Whitewing and Birchfall. Whitewing is a white she-cat with green eyes, and Birchfall is a light brown tabby tom with amber eyes.
Birchfall.
Her father is Birchfall (Dustpelt and Ferncloud's son) and her mother is Whitewing (Cloudtail and Brightheart's daughter).
No, they're Birchfall's.
Whitewing and Birchfall
Yes, Ivykit and Dovekit.
Birchfall
This is not likely. The book Sunrise implys that Birchfall was the father of Whitewing's kits when Birchfall said; "We've called the gray one Dovekit, and the tabby and white one is Ivykit." Although this does not explicitly prove that Birchfall is the father, it certainly implys it and the books have given us no reason to believe otherwise.
Maybe, because they were sharing tongues. But then Ashfur got killed by Hollyleaf and Whitewing had kits with Birchfall.