For some reason many vegans are very dogmatic and religious about it. Many of them will outright distort, lie and fabricate claims, and will viciously attack critics with all the fallacies and tactics in the book.
Like all other religious people who try to impose their dogma onto others, they will often try to masquerade their own principles behind good-sounding reasons, which are nevertheless just excuses.
For example, one of the most common excuses is that they are vegan because of the mistreatment of farm animals. They will go on and on with sensationalistic stories and pictures of extremely poor treatment of farm animals. An appeal to emotions, essentially.
But if that were the reason for their veganism, and if they are consistent about it, then surely they wouldn't have any problem in eating animal products that have been fairly and humanely produced?
Will a vegan eat fish? No. Because of cows and chicken being mistreated somewhere.
Will a vegan eat eggs that have been produced in what's essentially a chicken paradise, where chicken live a luxurious life that's much better than they would ever live anywhere else? No. Because cows and chickens are mistreated somewhere else.
Will a vegan drink milk that has been produced in what's essentially a cow paradise? Of course not.
The "animals are mistreated by the food industry" is just an excuse. They are seldom consistent about it. If that were truly the reason for their veganism, then they would have no problem in eating animal products that has been produced humanely. But they don't, because it's just an excuse.
The other major reason they always give is the claim that a vegan diet is healthier. These claims are often ridden with exaggerated and outright fabricated claims about the health hazards of meat.
(Processed meat can be somewhat detrimental to health, but meat itself is not. For example, there's a widespread misconception that red meat is unhealthy. This is false. Processed red meat may have health risk factors, but unprocessed red meat does not. In fact, when you study more remote and isolated populations who eat principally unprocessed red meat, you'll find that they are in general very healthy, with little to no coronary artery diseases and other such problems traditionally associated with meat. It's not the meat itself that causes this; it's the kind of meat.)
And a vegan diet is not somehow automatically healthy. I can live with a 100% vegan diet, and become extremely sick from it in a year. The diet still has to be varied and contain all necessary nutrients in proper amounts, or else you will suffer from deficiencies.
This is in no way different from normal food. You can have a very healthy varied diet containing animal products, and suffer no ill consequences from it. Veganism all in itself is no guarantee of health.
Moreover, a purely vegan diet tends to lack certain nutrients (like certain vitamins) because those are primarily found in animal products, and extremely rarely in plant products. If you live on a 100% vegan diet for years without taking care of getting those certain nutrients (which you won't get unless you are specifically aware and use supplements), you will suffer for deficiencies.
It is, in fact, easier to have a healthy diet containing animal products than a purely vegan diet. The traditional wisdom applies: Some meat, veggies, milk, eggs and so on, in a reasonable proportion each, using common sense, and you are quite unlikely to suffer from any deficiencies or negative health effects. You generally don't even have to know about nutrition or what kind of nutrients can be found where. Not so with a purely vegan diet.
The thing is, some vegans are really, really dogmatic about it. They are really obnoxious and resort to outright lying and distorting in order to defend their religion and to attack critics. Some of them are outright sociopathic. (I could point to concrete examples that you can find eg. on YouTube, but I'm sure you can find them with a bit of searching if you are really curious.)