We are descended from a lot more people than most of us can imagine. That shouldn't be all that surprising when you consider that ancestry is exponential. Each of us is 10 generations removed from 256 pairs of seventh great-grandparents. By the time we are 15 generations removed, we are descended from 16,384 couples. Folks in such areas tended to marry neighbors and cousins.
Family trees work both upward and downward.
If going back 16 generations means that one is descended from 32,768 people, think how many descendants each of those ancestors has. In researching only one line from my wife's family, I have uncovered more than 1,500 people descended either by blood or marriage from a 16th-century Englishman. Assuming he is representative of most heads of Elizabethan English families, multiply those 1,500 descendants by the 32,768 people representing 13 generations, and one is faced with the possibility of being related to more than 49 million people. Using that arithmetic, one could easily be related to a friend, a neighbor, a co-worker, or that cute young thing you've been working up the courage to ask out on a date. So if you were wondering why we tend to marry people who look and act a lot like ourselves, this might offer at least a partial explanation.