It doesnt, its pretty normal to be vegan. Do what you know feels right.
I think some of it is people Know it is the right thing to do, and veganism aligns better with their moral stance, but they are not ready to try it. Or they perform mental gymnastics as to why its okay and you are wrong. When people give you a hard time about veganism, its really them having an internal battle with themselves, and you just happen to be the one triggering it and thus its your fault.