Why cow is sacred to hindus

All Indians and many non-Indians know that cow is sacred to Hinduism. But if you ask why, then nobody will be able to give you all the reasons. Everyone will say a point or two but there are many more reasons why cow is considered as sacred to Hinduism. Let us see some of the major ones.
  1)    Main reason is teaching of lord Krishna. Note that lord Krishna was raised as a cow-herder. Now, being a god, why did he choose himself to be raised in a low-caste cow-herder family? Answer is he wanted to stop the people from doing the sin of killing cows. Before lord Krishna was born, slaughtering cows or cow sacrifice was followed in some kingdoms especially by low-caste tribals. Moreover, in some kingdom, beef was eaten by kings, his community and people. So, it is lord Krishna who through his Maya taught the people why the cow should be treated as a mother. It is said that whenever lord Krishna would play his flute, all the cows would come running towards him and would constantly look at him. Within a minute he plays his flute, he is surrounded by all cows and other animals. Amazed by this, his villagers asked the reason behind this. Lord Krishna said smilingly that “Among all the animals god created, cow is the only animal that has been given utmost feeling of love and compassion. They surround me because I love them and they love me back.”

  2)    He also said that cow milk is the best alternative to mother’s milk and is more nutritious of all foods and so we should call cow as ‘gavmata’.  Gav mata translates as Gav-Cow, Mata-Mother.
3)    This is why lord Krishna is also called as bala-gopala, (the child who protects the cows)
        Another of Krishna's holy names, Govinda, means "one who brings satisfaction to the cows."        
  4)    Waste of any animal is not as much useful as cow milk. Cow-waste was used extensively in agriculture those days and even today in remote areas. The cow's dung was a neccessary fuel and fertilizer and the bull is a beast of strength valuable to man 

  5)    Cow is more sacred because it is a reincarnation of human souls and above human souls. Many Hindu’s choose a vegetarian diet because of this belief. 
6)    When the cow dies his natural death, then it skin can be used a leather to fit our daily needs.
      7)    The gestation period of a cow is 285 days which is ~ equal to 9 months and the gestation period of human is 266 days, hence cow is mammal which keeps its young one as human do and also its milk being very thin similar to human milk is the only substitute to human young one if mother is unable to feed its milk and hence it is considered as second mother after real mother and hence cow is sacred like mother.
    8)    The five products (pancagavya) of the cow - milk, curds, ghee butter, urine and dung - are all used in puja <http://www.religionfacts.com/hinduism/practices/puja.htm> (worship) as well as in rites of extreme penance. The milk of the family cow nourishes children as they grow up, and cow dung (gobar) is a major source of energy for households throughout India. Cow dung is sometimes among the materials used for a tilak <http://www.religionfacts.com/hinduism/things/tilak.htm> - a ritual mark on the forehead. 
9)    The cow has a special role in the Hindu mythologies; Kamadhenu <http://www.kamat.com/kalranga/women/shashwati/5451.htm> is a wish-fulfilling cow. A cow is also depicted as vehicle of several deities. 

 10)     Cow can also be used for purposes like ploughing of agricultural fields.
11)    In many Indian villages Cow Dung is still used as fuel to cook food.
12)    All they provide cows are considered the mothers to humans and therefore should be treated with respect and never eaten. In India, the cow is believed to be a symbol of the earth - because it gives so much yet asks nothing in return. They are guileless in their behavior and from them flow sacrifices. The cow is the principally the supplier of milk, a symbol of purity, piety, serenity, peace and motherhood  

Below are some of the other facts related to cows and its worship in india
  1)    Today, in heavily Hindu nations like India and Nepal, milk continues to hold a central place in religious rituals. And in honor of their exalted status, cows often roam free. Indeed, in some places, it is considered good luck to give one a snack, a bit of bread, or fruit before breakfast. On the other hand, a citizen can be sent to jail for killing or injuring a cow
      2)    Traditionally, Indians had cows in every household. They were part of the family, with names and personalities. Just like one would not hurt/eat their pets, the Indians did not hurt the cows and respected them.
3)    According to many Hindu texts, giving service to Cow is good deed, so providing fodder to cows is also considered sacred.
      4)    Despite their sacred status, cows don't seem much appreciated in India. Visitors are often surprised to see them walking neglected around city streets, living on garbage from the gutters. But the cow is honored at least once a year, on Gopastami. On this "Cow Holiday," cows are washed and decorated in the temple and given offerings in the hope that her gifts of life will continue. There is also a festival for cows called 'mattu Pongal'.
5)    In ancient India, oxen and bulls were sacrificed to the gods and their meat was eaten. But even then the slaughter of milk-producing cows was prohibited. Verses of the Rigveda refer to the cow as Devi (goddess), identified with Aditi (mother of the gods) herself.
6)    Even when meat-eating was permitted, the ancient Vedic scriptures encouraged vegetarianism. One scripture says, "There is no sin in eating meat... but abstention brings great rewards." (The Laws of Manu, V/56)
7)    In India some priests can also be seen drinking Cow Urine, which some consider sacred and helps in removal of many diseases.
8)    In Hindu Mythology there was a cow called Kamadhenu...is was the pet of a ancient Muni / Rishi / Saint. It has the power of several armies and could give anything one asked for. Hence, the Hindus pray the cow.
9)    Once Lord Krishna was in the forest and was very hungry, he befriended a cow that gave him nourishment in the form of milk. We in turn respect the animal (the cow) that provided necessary nourishment for our Lord.
      10)    The cow was one of the outputs from the Churning of the Milk Ocean according to Hindu mythology (the same incident in which Vishnu assumed His second avatar as a turtle to support Mount Meru from beneath so that the Mount could be used as a churning tool.) Mythologically, hinduism owes a lot to this churning as some of the holiest and pious things like the Vedas, the Brahman, Goddesses Lakshmi, Saraswati etc had materialized out of it. The cow is therefore a gift to mankind from this churning just like the Vedas and other things. 
11)    Cows are considered wealth equal to gold and 'go-daan' or gifting of cattle was a part of every sacrifice and ritual. Cow is therefore an auspicious and valuable gift that brings prosperity to the receiver.