For me it was health and environmental awareness. I suffer from psorasis and had read that cutting out dairy in particular might help.
I did Veganuary this year having never been vegan or vegetarian before. I didn't expect to last a week but 5 months later I am still going. I have lost 13kgs, sleep like a baby and have more energy than I ever have. Skin is improved but not the magic cure I might have hoped for.
I actually don't have an ethical problem with the principle of using animal products but the rate and the way in which we consume them is unsustainable IMHO.
One of the things which frustrates me is that people feel it is a private club which full of judgy people. Most vegans are lovely friendly people who are happy to help if you want more information but don't judge you for your choices.
I am in a vegan/ veggie support group as we aren't in the UK and options are more limited here. We also have meat eaters in the group who just want to explore it and try some new ideas. I joined months before I became vegan and was really welcomed.
I have noticed that often when I tell people I am vegan (only if it comes up) people are often quite agressive or defensive about it. I am sure because they assume I am judging them and I am absolutely not!
Ultimately, the environment is going to be better served if everyone cuts down on animal products, rather than a small number being really restrictive, so every little bit helps.