I was vegetarian originally because the idea of animals being killed for food horrified and disgusted me. When I learned that egg laying hens and their brothers/fathers/sons, and dairy cows and theirs are all brutally slaughtered just like other "food animals" as an integral step in the process of "producing" eggs and dairy products, I knew I needed to be vegan.
If I wouldn't consume a chicken pot pie, hamburger or veal chop for "ethical reasons", how could I be willing to continue funding the slaughter of those animals in the name of dairy or eggs? What I had always thought I "meant" by choosing vegetarianism, I could only practice by getting rid of *all*animal products. And once I got that, going vegan was a piece of cake.