1. Raw milk can include germs or viruses that make you sick.
2. Raw milk spoils quickly. Pasteurization is what delays rapid spoilage. Before pasteurization most milk that people consumer was produced very close to home and consumed immediately (e.g. you milked the cow in the morning, and drank the milk for breakfast). The ability to process and transport foods is what allowed cities to get large (imagine if every apartment in New York had to have its own livestock in the backyard).
3. Unless you milk the cow on your own farm, the raw milk you acquire is not fresh. The person asking the question needs to differentiate between drinking fresh milk from a healthy cow, or milk that has been processed, bottled and transported to market, possibly from a cow whose health might not have been closely monitored.