The Continuing of "Friends"

Why is it that people have the saying "I can have more than one friend" when they can't seem to be friends with more than one person?

I have a friend who seemed to be jealous when I had other friends. However, I never left this friend out. When I was friends with others, I was still friends with the first friend. However, I have other friends with that all of a sudden no longer spoke to me after becoming friends with another person. When I asked what was the deal, had I offended them or what not, I was told "I can have more than one friend." To which I replied, have all the friends in the world, just don't forget the ones you got. However, the friends they had, all of a sudden distapeared. I wasn't the only one that got left out, so this really aint no pitty party, just trying to understand. If anyone has any comments on this, it would really be appreciated. I just want to know, why some people say "I can have more than one friend" but yet can't seem to be friends with more than one. Comments welcome