A few love it. Some women are very interested in it, to see it, touch it, smell it and taste it. Of those who do like to swallow semen most of them like it because the man loves it.
Men have the basic need to be "accepted" by you. His semen is him, at least it's from him. He feels hurt if you "reject" it. It's like you are spitting out him.
It brings you closer to him. He feels more protective of you and respects your doing it for him. Also he knows he is with you because he is in you. It's good for him that you do it.
But, if a man has a disease that is contagious and is present in the semen then you could get sick and need to see a doctor. You can get all STD's and HIV through oral sex.