OK, lessee if I've got this right: humans, who already had domesticated cattle for meat, cheese, butter and fermented milk, selected a gene that would allow them to digest whole cow milk, in order to have better nutrition, by being around milk. Not drinking it after a certain age, just by being around it. And this tolerance for that dairy product, unlike those other dairy products, made them candidates for survival, while those who... didn't stand close enough to the whole milk... died before reproducing. It makes perfect sense. Those creationist idiots will probably make fun of that logic.
Humans didn't do anything. A mutation just happened in an individual, and the offspring of that individual were able to receive better nutrition than the offspring of other individuals without the mutation.
Standing close to whole milk didn't do anything -- the mutation would have occured in any event. However, in a culture without cattle, the mutation would have not been beneficial and would have conferred no benefit on individuals with the mutated gene. In that circumstance, the gene would have probably disappeared. Selection is what favors a particular random mutation within a given environemnt.