To become vegan is to respect yourself, your body, and your mind. Putting good food in your body is to treat it with high regard. After a while, you expect the same appreciation from the people you know. You may more easily notice respectful behaviors towards you and discern actions or words that disfavor who you are or who you want to be. If you've been lenient towards subtle ridicule in the past from people with whom you have a relationship upon which you place some value, after becoming vegan, the value of that relationship may wane.
When you become vegan, you are caring for yourself, the planet, and animals. Your compassion enlarges for yourself as well as for the greater community and all life. Realizing that some people who you know are dispassionate about the life around us may make you reassess how much longer your relationship with them will continue.
This happened to me. People who I thought added value to my life, seemed less important to me after becoming vegan. I went through a transformation, while many friends stayed the same. It became harder to connect with some of them, and as a result, those relationships ended, just as new ones emerged. Friendships are important, but some will change as you do.