Whenever I start to get frustrated, I try to think back to when I ate meat - I honestly didn't know the truth back then. And I'd never really thought of animals being *killed* such was the disconnect I had with my food.
Very very few vegans have been that way their whole life, almost all of us ate meat at some point, and it's not because we were bad people. For most of us there was one person or moment that made us stop and think "what AM I doing?"
In my case it was meeting both Lyn White and Jill Robinson on the same day and us all having a genuine discussion about farmed animals - they left SUCH a positive impression on me, they didn't look down on me for not being aware of the truth, they simply let me know why they didn't eat meat - that was enough, I haven't eaten meat since that day.
If I ever get questioned about my diet, I take the opportunity to try to leave a positive impression, like Jesse said above. And I secretly hope I can be the moment that makes that person think "what AM I doing?"

I agree with Jane. Not everyone was born vegan or vegetarian. You have to remember that you are changing people's lives, or at least trying. Alot of people genuinly believe they are making the right choices for their bodies and don't have an issue with what they consume because that is what they have been lead to believe their whole lives. One person saying otherwise is not going to change their minds immediatly, let alone one person getting fustrated that they don't have the same point of view and decide to get abusive/disregard that person's opinion because of it.
It took me two years just to become vegetarian. And personally, it was a long process. I've grown up in a rural area where farming is the lifestyle most people undertake, the majority of people eat what they grow here, so when I started questioning the food on my plate, people looked at me like I was asking ridiculous questions.
I was pescetarian for a year before I put my foot down and said no, fish are animals too. And how I got to that point? People like Jesse and Jane who took the time out to explain their point of view and not attack me for my opinions, little did I know I was just repeating what had been drilled into me since I've been born, we "need" animals to live, and if you don't eat fish, you won't receive enough protein and will become critically ill. That and the information people like Jane and Jesse kindly and respectfully led me to, so I could make the connection myself if I didn't want to take their word for it. Part of the reason it took me so long was because I had others on here attacking me because I still ate fish, others who couldn't understand how I didn't make the connection, and transformed their anger at the industry towards me, as I was a living example of why the industry gets away with what they do.
This is something important we need to try and remind ourselves, we are changing people's lifestyles, and alot of the arguments we are given against our diet is what those people know and believe, be kind, open-minded, and patient.. because the majority of people, like me, if treated that way will make that conncection themselves.
It took me two years to make the connection to go vege, and already again thanks to people like Jane and Jesse, I plan to go vegan as soon as I have the resources. People who attacked/got fustrated at me because I couldn't see what they saw yet left me wanting to shut down, put my foot down and ignore everybody.
Ok so that was a massive tangent.. hope it makes sense! n_n