Hi. You are not selfish to consider that you do not want children. This decision to have or not have a child is up to you, not your mother. However, you are very young. You do not know what may change in your heart or soul or mind. When I was in my early 20's I did not want children either. By the time I was in my late 20's it had become a great desire of mine. I did a lot of growing up in my 20's.
You may not see it happening, but you may change your mind about this one day. Vasectomy is basically considered unreversable. Now if you were 35 and said all the same things, I would probably totally agree with you.
Surgery is drastic. Give it a lot more thought. There are other methods that will do until you get older.