Could you look an animal in the eyes and say to it, 'My appetite is more important than your suffering'?
So in 1987 I gave up all animal products and became a vegan. Simply so that I could eat and live in accordance with my beliefs that animals have their own lives, that they're entitled to their own lives and that contributing to animal suffering is something that I don't want to be a part of.
I believe that to knowingly commit actions that cause or condone suffering is reprehensible in the extreme. I call upon you to be compassionate and treat others as you want to be treated. If you don't want to be beaten, imprisoned, mutilated, killed or tortured then you shouldn't condone such behavior towards anyone, be they human or not.
If we base our belief systems on the humble assumption that the complexities of the world are ontologically beyond our understanding, then maybe our belief systems will make more sense and end up causing less suffering.
Mainly I'm a vegan because I like animals, and I don't want to be involved in their suffering. Also, it's better for my health and for the environment.