We naturally have Lactase enzyme when we are infants, which helps us digest our mother's milk. But most people in the world experience a decline in this as they age - about 70% of the world. The ones who typically tolerate Dairy long term are of Northern European descent. Thus, lactose intolerance could be considered a normal evolution. Digestive issues can contribute to lactose intolerance as well.
Raw dairy still has Lactase enzyme, which helps to digest the Lactose sugar naturally found in milk. This enzyme gets lost in pasteurization, part of the process for mass production and legality.
Here's a good article that explains this well: http://tummycalm.com/lactose-intolerance.html
Infant lactase digestive enzyme drops placed in the milk or formula can help the baby digest the lactose better. Signs of improvement could be less gas and moodiness.