Veganism Is The Moral Baseline If You Respect Sentient Life
The root of veganism is having basic respect for sentient life (the animal kingdom), the environment which provides all life forms their home and sustenance, and knowing that it isn't okay to discriminate and cause someone unnecessary harm and death and rob them of the autonomy they should have over their life. When you live vegan you live in a way that excludes exploiting others as much as possible. You avoid doing things that directly or indirectly cause or promote harm to others as much as practically possible. Veganism rejects human supremacy, objectifying life, and all forms of oppression that grow out of delusions of superiority such as speciesism. Speciesism is the belief that humans matter more than other species and that the trivial differences that humans possess give us a right to treat other animals less than the persons who they are. Speciesism is also when you believe certain non-human species deserve better and more fuller lives than other non-human species; for example, thinking a dog has more of a right to their body and life than a cow.
Although the word "vegan" wasn't coined until the 20th century, there have always been humans who were vegan and rejected using and killing others for their own benefits. Using others for their life and their body has never been mandatory and eating other animals has always been a choice, even when it's a matter of survival. In the modern age, we have way less excuses to exploit and kill and consume others than ever before in history.
The word and lifestyle of veganism came about to try to bring about the emancipation of other animals exploited by human beings. The definition for vegan goes:
"A philosophy and way of living which seeks to exclude—as far as is possible and practicable—all forms of exploitation of, and cruelty to, animals for food, clothing or any other purpose; and by extension, promotes the development and use of animal-free alternatives for the benefit of animals, humans and the environment"
There's a widespread misconception about vegans, born out of hate for them, that veganism is about being perfect. People want to make vegans look crazy, and pretending that a large group of people thinks they are perfect (and therefore hypocrites) is a convenient way to make vegans appear crazy and excuse yourself from not improving as a person at all. Unfortunately, vegans living in the modern age still do cause harm, but it's a guarantee that they cause significantly less harm than their pre-vegan self. By being vegan you reject consuming animals, wearing animals, using animals for entertainment, imprisoning animals in zoos, killing animals needlessly, using them as models for experiments, using them for sacrifices, and the list goes on and on.
A vegan diet is the diet of a vegan, which is exclusively plant-sourced by default. Vegans can technically eat things and wear things that look like they could've come from animals, just as long as animals weren't intentionally harmed in the making of these products and as long as these products don't send the wrong message and promote something derived from animal enslavement and killing. Most household cleaning products, cosmetic products, and hygiene products have been needlessly tested on countless animals of various species and vegans purchase and/or make the large variety of versions of these products that didn't involve agonizing experiments being performed on non-consenting individuals. Many things we take for our physical and mental health have been tested on animals and some of these products contain animal-derived ingredients, but luckily there are plant-based and non-tested versions of many of these things that will help with your same goals. If you absolutely need something for a condition you have that isn't technically vegan, well that's one example of when the "practicable" part of the definition of veganism comes into play.
When you are vegan you realize there are endless alternatives for everything from your non-vegan days. The world of plant foods is so big it's beyond comprehension and most, if not all, of your favorite foods and meals that involved animal sources can be replicated just as well or even better with vegan friendly versions. There's also nothing inherently dangerous or unsafe about a vegan diet. The largest association of nutritionists and dietitians from around the world, the Academy of Nutrition, has even stated that a balanced vegan diet is healthy for humans in all stages of life. If you love the look of leather you can buy vegan leathers that are made from many different things other than animals. If you like being in close proximity to animals who you wouldn't normally see or have as companions of your own, instead of visiting a zoo with dejected animals you can visit a farm sanctuary where neglected and abused animals are given a second chance at life and get to be themselves.
Veganism shouldn't be purely about rejecting and avoiding causing harm, exploitation, and killing to others but it should also be about standing up for truth and justice and helping whenever we possibly can by educating others as well as by trying to offer relief and prevent death by any means necessary for all oppressed beings. Veganism in itself is a good thing but it's an inaction, if you truly want to help animals you need to get active in some way or help those who are active for animals, whether that's helping out financially or in some other manner. Most genuine vegans say the only thing they regret about going vegan is not going vegan sooner than when they did.