Well to put it really briefly (and these are only a few arguments for being vegan): The (mainly) Western world's huge consumption of dairy products means that dairy cows are exploited and generally live only a quarter of their natural life span often in a degree of pain; we rely on developing countries to provide crops to feed ourselves and our cattle, rather than allowing them to grow food for themselves.
The amount of animals bred for the meat/dairy industry contributes significantly to dangerous levels of gas (I'm not a scientist) creating hole in ozone layer; most people are allergic to dairy milk but dont know it; animals are still kept and killed inhumanely, even Free Range products are linked to this; meat/dairy doesnt give you anything you cant get from a vegan diet (apart from heart disease etc.); there's no reason to suggest it's 'natural' to eat meat/dairy; life is sacred, violence is wrong, eating eggs/dairy just contributes to the whole meat industry ...... and I'm not just talking out of my own arse, I once worked in an abbatoir, before I saw the light