The main goal in Krishna consciousness is to "go back to Godhead." Basically to get to the Kingdom of God. But that is not replete with selfishness, at least nit shouldn't be. That is why you will see devotees on the streets singing "Hare Krishna," when we do this, it benefits our spiritual lives, but at the same time it benefits other peoples' (and animals') spiritual lives as well. That is the main reason we do it - to spiritually advance others, even though they are usually unaware of it.
The same is true with the cow and her milk. Let me explain. Devotees offer food to Krishna; the food then becomes spiritualized, taking away the karmic reactions of killing the plant. (We believe all living things should live a complete life span, but in order to survive we must kill, so we kill plants instead of animals because Krishna will not accept an offering of dead flesh. Doing this washes away the karmic reaction associated with killing.) Now, as far as the milk is concerned, the cow is obviously a lower species as compared to the human, and in Krishna consciousness we would like every one to be spiritually more advanced in their next life. So wouldn't it be nice for there to be a way to "reward" our mother cow for her generous giving? Well, there is. By offerering her milk to Krishna, she is automatically spiritually advanced in her next life. That is the best gift that you could give apart from taking her in and giving her a slaughterhouse free home. Even better than refusing to drink her milk, thinking that you are boycotting cruelty.
People who do that often think only about the present, "I'm going to stop the cruelty of today." Well what about the spiritual life of tomorrow and beyond? If we are Krishna conscious, we should think more of the spiritual future than the material present. It only makes sense.
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