People can only understand emotions in terms of their own experience with them. Most would agree that we do not like to suffer, and so we logically conclude that other people don't like to suffer. So the question is, can other non-human life suffer? And if so, which life?
If we can agree that human suffering is wrong, then the we must accept one of 3 possibilities:
1. Non-human life cannot suffer, therefore it is not wrong to harm non-human life and it is acceptable to eat anything (the most common stance among people)
2. All life can suffer, therefore it is wrong to eat any life. This stance is impossible, since we need to eat organic life to survive
3. There is some arbitrary point at which life suddenly gains the ability to suffer. In this case, a person must draw a line as to where the suffering begins. Most people with this stance draw that line close to the human branch of evolution. Some will not eat read meat, others will go further to birds, and then fish
Personally, I find the 3rd stance (Vegetarians and vegans) to be irrational. I'm not sure how a person can say it is wrong to kill a cow, but acceptable to rip a weed out of a garden. It is wrong to eat a fish, but acceptable to eat a mushroom. The difference, biologically, between man and plants and other forms of life is not as great as most people think. I find this stance hypocritical. That of course does not mean that I would recklessly harm non-human life because it is unimportant - man is still a part of the Earth and its ecosystem, and our survival is contingent upon us taking care of the earth and everything upon it. But I don't think eating animals is going to destroy the ecosystem, as long as we take care not to hunt to extinction. Plus, meat tastes really good