The basis for my theory revolves around my brother who, as a life-long dairy consumer, made the decision to quit eating everything milk-based about 6 months ago. He claims it improved his energy levels, skin health, and digestion; which is great.
The interesting part, is that it seems he induced lactose-intolerance during his abstinence. Eating cheeses, creamy dressings, milk, or anything of the sort will cause him strong stomach discomfort.
My theory, which is shared with me by many others, is that humans were not meant to drink growth-promoting breast milk made for baby cows.. It could be that traditional lactose tolerance in humans begins developing early enough in life, that average people must acclimate to dairy.