Biology taught in Western schools in the 1950’s implied that meat and dairy were essential and had to be eaten every day in the same way that one and one equalled two. No one asked what happened if you didn't. It was inconceivable.
And yet many communities have lived for millennia on a largely plant based diet. Until recently, most of the world only ate meat on special occasions like carnivals (carne - Latin for meat). A small fraction of the meat, egg and dairy consumed nowadays.
health
The increase in meat and dairy consumption over the past century or so is a main cause of the epidemic of Western lifestyle diseases like cancers, osteoporosis, diabetes and heart, vascular diseases. Also auto immune diseases like arthritis, Lou Gehrig's Disease(ALS), Crohn's, Parkinson's, Alzheimer's, fibromyalgia, systemic lupus, scleroderma, Chronic Fatigue Syndrome(CFS) and endometriosis. One of these illnesses is usually followed by another.
Animal based foods are more contaminated by dangerous micro-organisms during farming and processing. The by-products of digesting and metabolising them are damaging.
Eating further down the food chain means less heavy metal and pesticide residues.
Animal products are estimated to be the largest risk factor for cancers and some cancer clinics are putting recovering patients on a vegan diet after radiation and drug treatment.
Plant-based foods (especially raw and fresh) protect against most western lifestyle diseases.
Health, feelings, vitality and fitness usually improve on a balanced vegan diet. Typically no more recurrent colds or flu, headaches, runny nose, depression, lack of energy. Some effects are immediate, others take months.
Plant foods contain not only oils carbohydrates, protein and amino acids but also antioxidants, enzymes and countless other nutrients. Some of these nutrients have been found to be useful for health and some are essential.
Components of plants including plant DNA interact with animal metabolic pathways in complex ways which have arisen as they evolved together over millions of years. Knowledge of these is increasing in western science but the mechanisms are still pretty much completely unknown.
Research shows that plant foods are more efficient sources of energy and nutrients and easier to digest and utilise but all their nutrients and how they work are yet to be discovered. In the meantime personal knowledge and judgement are required to select food.
adaptation to plant food
Humans have evolved with the anatomy, physiology and biochemistry of herbivores. This includes the vision, metabolism, pH, enzymes, teeth, behaviour and the long gut and digestive system of herbivores. Plant food is a better fit in most ways.
Humans can survive on a diet of mostly animal products if some are eaten raw and fresh or part decomposed and offal is used. Meat and milk can be partially decomposed by micro-organisms to predigest them. Life span is shorter and health not as good as in vegan communities.
compassion
Deep down most people are uncomfortable with the exploitation of other animals even if they haven't consciously thought this through. Most people have to steel themselves to kill them.
It is not such a large step from the exploitation of other species to the exploitation of other humans. By exploiting other animals we are closer to exploiting other humans, overlooking their feelings and relating to them as a commodity.
The social contract assures that fellow citizens are not going to hunt you down or trick you into becoming their dinner. It requires compassion and trust. This contract is not universal and from stories of its break-down here and there, cannot be taken for granted. Killing other animals flags the possibility that it might not necessarily be reliable. Usually this can be kept below conscious awareness and ignored.
Just being vegan may bring to the awareness of others in the room the possibility of death at the hands of someone else and guilt at benefiting from animal slaughter and so might be an existential threat and trigger a strong reaction.
If someone reacts to you being vegan with ridicule or anger it may be to shut out the threat that the social contract may be fragile and keep feelings of guilt or fear at bay. The way you respond can alleviate or expand their fears. There is usually more happening under the surface than just concern or curiosity.
environment
Eating vegan reduces our impact on the environment. Meat requires more land and water to produce and is more destructive and polluting. The consumption of animal products is one of the main causes of deforestation, global warming and indirectly, hunger, starvation and war. The most effective thing an individual can do to minimise greenhouse gases is to not eat animal products.
Emerging industrial societies are eating ten times more animal products than their predecessors and hundreds of times more than some traditional communities. This trend does not fit with large growing human populations.
spiritual
Most religions including the major religions have traditions of vegan or vegetarian eating particularly in their meditative orders. Respect for other beings, communion with the natural world and emotional and intellectual clarity are among the reasons.
reasons
Westerners are increasingly switching to a plant based diet, usually after something happens to expand their compassion for other animals or motivates them to save the planet. For some it is a food preference. For others it is to recover from serious illness or stop it worsening.
cuisine
Changing to a plant-based cuisine opens up a previously unexplored range of ingredients and subtle food combinations once the dulled tastes and after effects of animal products that dominated meals are out of the way. As senses of smell and taste return there is a richer and wider variety of ingredients to choose from.
adjusting
The body may take a few months or a year or so to fully adjust to eating vegan depending on previous diet and how big the change is. Different digestive enzymes are secreted and there are changes in the metabolic pathways as energy comes more from sugars and carbohydrates and less from the less efficient and more toxic breakdown of fats and protein.
The degradation products of plant foods are easier on the metabolic and elimination systems.
Whether someone actively seeks out plant foods or just cuts out animal products they usually eventually feel better eating vegan and come to prefer the tastes. Non vegan foods become less enticing or repulsive and plant foods become an unconscious preference rather than a calculated choice.
Plant foods have more flavours and textures and nutrients so adding them to the kitchen is easier than agonising over cutting out animal products. Animal products swamp other tastes in food. An exciting new world opens up when they are out of the way.
Usually after a few weeks of any diet a return to a previous diet feels strange and less attractive because tastes acclimatise to food eaten over the last few days particularly to salt, fat and sugar.
After decades of eating vegan, animal products are usually difficult to digest and they smell and taste unpleasant and bring a noticeable drop in performance and feelings of well-being.
Research shows that vegetarians tend to measure higher in intelligence than the overall population without suggesting whether this is cause or effect.