I would advise against telling someone who they are or why they feel a particular way, without at least some minimal level of acquaintance.
Meat comes from the corpses of animals slaughtered on our behalf. Selfishness does not come from eating it, or from insufficiently valuing the environmental impact of it, or from getting annoyed with people that have made it a cornerstone of their personal religion.
I don't feel challenged at all by vegans, in the same sense that I am not challenged by missionaries knocking on doors or distributing flyers. When they speak, and I detect the emotional investment, I tune out, because I have learned long ago that one cannot argue with religion. I don't want a religion; you can't convert me by preaching.
So when someone tells me I am selfish, knowing little of me beyond the words in my initial post, I have exactly the same emotional reaction as with someone telling me that I will go to Hell for being atheist. It only makes sense to a believer, and I am not one. I tuned my bullshit detector with years of pointless arguments with kooks and crackpots, and no one but an expert is going to slip a rhetorical cheat past it.
If I am selfish, by definition, I wouldn't care whether other people think I am selfish. Ergo, attempting to sway someone by suggesting they might be selfish is implicitly dishonest. It would only work on someone already concerned that they might be too selfish, while not knowing of any specific reasons why that might be so. By introspection and by comparison with acquaintances, I have reached the conclusion that I am not selfish, or at least not more than one standard deviation above the median level of selfishness. I am stubborn and skeptical; sometimes it comes out looking similar. Since I know myself better than anyone else does, whenever someone comes out and says I am selfish, my bullshit detector pings, and I look for the ulterior motive.
Aha! The person making the claim has no specific interest in me or my prior claims, but is using me as a prop to preach to the more accessible listeners in the audience, and to their own choir. They are treating me exactly as I treat a piece of meat--a thing to be used to advance their own ends, rather than something of inherent value. That's hypocrisy, of a sort, and I dislike it in vegan preachers, and every other kind of preacher.