Why Do Hindus worship the cow?

Before we should know that why is the cow so important in India.

What is special about cows? 

Agriculture is not only the art and science of cultivating the soil and growing crops but it is worship for Indian farmers. Animal husbandry is also part of agriculture. Agriculture and animal husbandry are an integral part of Indian Culture.
Cows have always been a part of Indian agriculture and farmer families. We get the best quality products from the cow like milk fertilizer for fields. Indirectly be get cured, buttermilk, Ghee etc also from the cow itself. The nature of cows is calm. This is why cow was among the first animals that humans domesticated.
Cows help to increase the revenue for the farmers and improve their standard of living. The animal waste can be used as source of manure for soil enrichment also a source of bio fuel.

Importance of cow in Shashra

Worship the cow is a way to show gratitude. The importance of cow can be understood from this verse of Gavopnishad.

घृतं मे हृदये नित्यं घृतं नाभ्यां प्रतिष्ठितम्।
घृतं सर्वेषु गात्रेषु घृतं मे मनसि स्थितम्॥
गावो ममाग्रतो नित्यं गावः पृष्ठत एव च।
गावो मे सर्वतश्चैव गवां मध्ये वसाम्यहम्॥

The cow, who gives us and milk, stays in my house forever. Ghee of cow should always remain you my heart and mind. Cow stay ahead of me. Cows stay behind me too. Cows are around me and I live in the midst of cows.
By honouring this gentle animal, who gives more than she takes, we honour all living creatures. The cow is also symbolizes dignity, strength, endurance, maternity and selfless service. The cow represents life and the sustenance of life.
We acknowledge this reverence for life in our special affection for the cow. At festivals we decorate and honour her, but we do not worship her in the sense that we worship God. 
The cow is so generous, taking nothing but water, grass and grain. It gives and gives and gives of its milk, as does the liberated soul give of his spiritual knowledge.
Veneration of the cow instils in Hindus the virtues of gentleness, receptivity and connectedness with nature.
Cows are garlanded at special religious functions as our homage to all animals.

Importance of cow

The generous cow gives milk and cream, yogurt and cheese, butter and ice cream, ghee and buttermilk.
It gives entirely of itself through sirloin, ribs, rump, porterhouse and beef stew. Its bones are the base for soup broths and glues.
It gives the world leather belts, leather seats, leather coats and shoes, cowboy hats.
Milk and ghee in particular are sacred gifts, which are essential elements in Hindu worship.
In India, more than 3,000 institutions called Gaushalas, maintained by charitable trusts, care for old and infirm cows.
And while many Hindus are not vegetarians, most respect the still widely held code of abstaining from eating beef. 
By her docile, tolerant nature, the cow exemplifies the cardinal virtue of Hinduism, which is non-injury.
The cow is so vital to life, the virtual sustainers of life, for many humans.