For a cow to produce milk she needs a baby. (Much like us humans).
So we need the cows that are used in the dairy industry to be in a constant cycle of pregnancy and birth. This obviously is not natural, so how is it achieved?
Well, a metal rod is coated in semen and forcibly inserted into the cow. She then falls pregnant. Upon giving birth, the mothers' baby is taken from her, causing a period of great trauma for both mother and child.
The milk that is then produced by the mother, the milk that she produces to feed the child we (humans) have just taken from her, is taken by us.
So to recap, the cow is raped and then her baby is stolen and then we take the milk from her.
What's more is that the average life expectancy for a cow is 20 years, the average life expectancy for a cow in the dairy industry is 4 years. We rape them, we kidnap and murder their child and then we deprive them of 16 years of life.
We do this every single day, all so that we can drink milk designed to raise healthy baby cows.