Namsakaram.
I have heard the following reasons in books and various speeches:
I am not quoting the exact sastric references..but only the essence, for exact verses What Kongil Pirati bhakta has given that link should serve well.
http://www.blog.gomataseva.org/cow-hindu-vedas-puranas/
गोमूत्रगोमयं सर्पि क्षीरं दधि च रोचना ।
षदंगमेतत् परमं मांगल्यं सर्वदा गवाम् ॥
Cow’s urine, dung, milk, ghee, curd and gorochana – these six are the most auspicious products.
Lord Krishna wants human society to follow varansrama dharma and elevate oneself spiritually.
Cows form the basic and most indispensable contribution in all varna and asharams
First during Brahmacharaya when one needs to study scriptures,milk helps to make the brain tissues grow finely to understand and also to have good memeory
Then when one gets upanayanam Panchagavya purifies one.
Then in Grhista ashramam- one needs to maintain oneself and the family with grains and also needs to perform sacrifices.
Cow dung and ox are essential for natural agriculture to provide required grains.
Cow ghee is the basic ingredient for fire sacrifice
Then for vanaprasta and sanyaasis milk forms the basic and simple diet which can provide complete nutirion.
Similarly for varanas, brahamanas need cow for their homam and yajna, vaisyas need cows for maintaining fields for their income and to maintain the society’s(which includes kshatriyas and Sudraas) basic needs .
So in summary without help of Cow, it is impossible to execute varanashrma dharma without which purpose of existence of earth would become void, so because of Cow’s immense service attitude Lord Krishna likes cow very much.
There is also another side point to note that after cow, next birth is usually human form. So Lord is happy and eager that the soul will be given better opportunity for self realization. So when a cow is killed prematurely it will have to take birth to complete its karma before being born as a human..this may be another reason why Lord Krishna dislikes violence to any being but more so towards a cow..
Adiyen