Something I have come across a number of times is the idea that compassion for animals comes as a result of choosing a vegetarian diet, not a cause. That is, in some cases, people choose vegetarianism for reasons other than not wanting to kill animals; the compassion/distate for killing/‘animals are people too’ mindset came later, once meat was out of their routine and out of their system.
That's not entirely my experience, but I can attest to the fact that my compassion has increased with my vegetarianism. When I first decided to go vegetarian, it was quite an intellectual decision; I decided I couldn't really justify killing animals for the sake of tickling my taste-buds.
Now, I could barely stomach killing an animal if my life were on the line. (For the record, I probably would kill for my own survival, but I wouldn't enjoy it.) And the idea of eating meat for any lesser reason is, well, unthinkable. Weird.