In the US it is considered bad form to date someone who is in your close circle -- to the level that it's called "friend-cest" where I am from.
Many Americans believe that by having your romantic relationships come from outside of your circle you're assured that you both remain independent, and it gives your relationship a sort of 'hybrid vigor'. That is, by drawing from a larger social area your relationship gains a greater vitality.
What's more, it is considerably less complicated socially -- e.g. there are no exes to worry about, or if you break up you don't have to worry about friends choosing sides.
It is also considered a very serious faux pas to date someone you work with. That will very often lead to both people being fired if it is discovered by management.
As a result, that means you need to look outside of your social network for romance. Hence, the existence of dates.