As kids we are quite ignorant - and this is in reference to friends.
But life kicks us hard as we grow older because we tend to get choosy. Some prefer fools, some prefer intelligentsia, some lookers, or just out of circumstance.
May be when we get older we realize that we dont really fancy the company of certain people but because there is a history - it is a given that they are your friends an will be for a long time.
Then there is an issue reg. gender.
Currently, I could go out alone with a female friend - no strings attached; but post marriage ( if I ever do get married ;) ) - would my wife not feel insecure about it. esp. if she is stuck at home, then my outing is out of bounds.
Currently, I can still make great new friends - but not far down the timeline, your emotional satisfaction and entertainment is taken care of by earlier friends - so the need or the room for a new friend is absent.
Even though that person may have great potential to be closer to you - neither do you have the time nor the inclination.
So what can all this mean? I believe this is very similar to what was a glimpse in Before Sunset. There will be people who you will have to pass unknowingly who could very well have molded your lives in different ways.
The consciousness of another person's entry into your life can allow you to value that person in different ways. Nothing may come of it - but it will be worth etching the entry in your mind.
This calls for reiteration - I write insane rubbish.