Why is the cow considered sacred in Hinduism?

The bull tills the land and churns the butter in rural India and is thus seen as a type of father who provides sustenance to the living entity. The cow provides milk which is used to make so many different types of food which also nourish the living entity and she is thus seen as a type of mother. The mother and father are always to be venerated. This is the basic idea. Beyond that, the highest expression of divinity within Hinduism is that of Krsna, the cowherder, who is God at play and expressing himself in his most intimate setting with his family and friends. Another name for Krsna is Govinda, the protector of the cows. Krsna loves his cows and calls them each by name as he takes them to the forest to graze. The cow is a very simple animal with minimal needs and gives so much to humanity. Because cows are so special to Krsna and he personally cares for them, it is quite natural for Hindus to hold the cow as sacred. Beyond that though, it is quite natural for all of humanity to appreciate the simplicity and humility of the cow and to protect and care for her as she does us.