Another question about ducks?

I have an inground pool and every year ducks come and we feed them. This year we have a female mallard and a male black duck couple. They come everyday, but there is also a mallard male that has been coming with them. The black duck does not allow him to come into the pool, and sometimes he chases him around. I guess what I dont understand is why the male mallard continues to follow this couple everywhere they go, when he is being chased, and is obviously afraid of the black duck (he never actually fights with him) I mean, why doesnt he go find another female? Why does he keep following the same one that already has an aggressive male?