For your first point, it varies. People are going to give you different reasons for their motivations behind this. Some people might just find diapers arousing, for example.
As for the second point, I don't think that's necessarily the case. While some AB/DL's may report lackluster childhoods, others had normal ones. So while a lack of affection may influence some it's not that way for everyone.
For me personally, my AB side does have a deep need for love and care. But then I've had a DL side since childhood that finds padding arousing. Mine is just one of many explanations the people on this site can give you about their desires.