While I no longer eat it, cheese kept me from being a vegan for many years. I had given up all other animal products, but as a midwestern boy with an Italian mother and grandma, I was raised on it, and found it the most difficult thing to give up. I figured it was alright since nothing had to die to make it...then I found out about rennet.
"Rennet is an extract from the fourth stomach of young ruminants, such as cows, goats, and sheep. This extract contains a number of enzymes which are designed to help these animals digest their mother's milk, and when added to milk, rennet will cause the milk to coagulate, forming the curds and whey which are so essential in the cheesemaking process." - Wisegeek.com
These stomachs are the by-product of making veal, which comes from slaughtered baby cows, sometimes only days old, depending on the type of veal. Yup, like these little guys - Unfortunately, most cheese is produced in this method, thus most cheese could not be made without slaughtering baby cows. That was enough to get me to stop eating it...well, and the fact that cow's milk is in no way fit for human consumption.
BUT, cheese lovers do not despair. If you can't get yourself to give up cheese, at least do the baby cows (and yourself) a favor by purchasing cheese made without stomach rennet.
Here is a solid listing of cheeses made with vegetarian rennet. I know the Whole Foods by me has their cheeses labeled as either "traditional" or "vegetarian" rennet. Of course it would be more truthful if it was "dead baby cow stomach" rennet, but oh well.