Having personally known several perfectly normal and sane vegans, maybe your "reasonable conversation" is a bit more combative than you believe. Vegans are just normal people. Some will be crazy. Some will be normal. If your experience with your hundreds of vegans you've met is 100% unreasonable, then you're definitely the problem. Someone choosing to avoid animal products for personal health or environmental reasons, or any other personal reason, is inherently not unreasonable. They might be unreasonable if they try to force their ideas on others, but defending their own choices isn't unreasonable. Tone down your confirmation bias and aggression, and you might find that just like every large enough group, people are still people and they vary.

Edit: for the record, I'm not vegan.