Lightning Dust didn't live in Cloudsdale as a filly
It would explain why Dust and Dash didn't know each other until they were cadets at the academy. Lightning Dust grew up in a different pegasus city and only visited Cloudsdale to participate in the flying races there.- This is compatible with the "Lightning Dust is Sky Stinger's sister" theory, and she grew up in the house seen in "Top Bolt".
At least one of Dash's parents had a bad childhood.
It doesn't have to go so far as Bow or Windy having had an Abusive Parent; it could be something as simple as that one of their parents was hard to please or slow with compliments, resulting in Bow or Windy becoming a "Well Done, Son" Guy or "Well Done, Daughter!" Gal to get their parent's affection. This story came up at some point during their marriage, and when Dash was born, both of them decided that their daughter was not going to have the same problem. They were going to show her, openly and continually, how much they loved her and how great a pony she was. Unfortunately, in trying to avoid one problem, they fell headlong into another... Bow and Windy had suffered at least one miscarriage before Rainbow.
Another explanation for their over enthusiastic behavior is the possibility that Windy has lost at least one other pregnancy, possibly leading her to believe she was not capable of baring a foal. So when Rainbow survived, she and Bow were ecstatic and smothered their daughter with love and support. It’s also a possible explanation as to why a couple who dote on their child don’t try for another one; they don’t want to take the risk.