One of her books talks about how we match up and who we choose.
We choose based on: Timing and proximity- to a great extent. Similarities are usually highly important for who we fit best- looks, ethnic, values, socioeconomic level, intelligence, meta emotional match- we like people who are like us, whether we are buying things or are selecting a mate. Whenever there is a mismatch, it is one more thing we need to negotiate. Keeps it from being too scary.
We want enough similarity to be comfortable and secure and enough difference to keep it interesting.
Think about the friends you have had the longest and the ones who are most interesting to you. Are they always the same ones?