Things run out right? Of batteries, or of interest, or usefulness. There are only a finite number of times that something can be used, like a dish scrubber, because eventually dirt will build up to overpower the cleaning action and it will become obsolete, or a dishtowel, which will become threadbare to the point of not existing. Or soap, that will really just disappear on you if you use it long enough, or shower often enough. 
Things that last longer though, like glasses can also run out because sometimes we just don’t want to see the same pair on our faces anymore. Or even if we do not tire of the same pair, our eyes are the other factor, and they will become more near or far sighted, so we have to change something. 
Friendships do that too, thought not all of them. Don’t get me wrong, I’m not bitter, well I guess I am. But we’re all bitter at something, right? Friendships/relationships sometimes will run out of juice. It will come with warning or none at all, and then that will be the end of it. Like an ipod that is fine for the two three years I’ve owned it, but one day looks and me and tells me to shove it, that it will no longer play any more music, denies that there IS any music to be played, when obviously I know it is still holding onto them, just hiding it somewhere. Well what can I do about it? It’s not like I can go into cyberspace and bully it out of him, if he says no, I could only restore it to its factory setting, and begin again, though with a lot less music.