Dandelion's mother left her when she was very young, and she became determined to find her. Even as she herself became the same age her mother had been when she had left, she searched, losing the few friends she had ever had to her obsession. Eventually, she got her answers.
Buttercup had left because she didn't want a pup or to stay in the Snow Sparks; instead she opted to live in a new pack far away. When she met her pup she didn't recognise her at first, and when she did she became hostile and chased her away. The knowledge that her mother abandoned her has destroyed Dandelion, who has turned inwards and barely even pays attention to Embers, the only wolf who's anything like her friend.