I would say that the individual grains of sand in the hourglass represent moments. The sand in the hourglass is constantly moving just like time is. When all of the sand has gone through, it starts all over again. If you sit at stare at an hourglass it seems to take a long time. However, if you are busy enjoying the things that are happening around you then it goes by quickly. This is kind of like life itself. It can go by in the blink of an eye.
It's an interesting simile. I chose to look at it as a time reference, if you watch closely as every grain of sand - every event in your life, passes by and is added to your history, then the relationship passes very slowly. If I get busy and forget to pay attention, then it passes quickly, just like some school years do for me.
Perhaps, the notion of an hourglass consisting of many sands as essential parts of its identity could be similar to a relationship containing many distinct parts. Each relationship contains many individual moments or components that help to formulate its identity and sense of self. The idea of each fitting into a whole is similar to an hourglass, where each grain of sand helps to feed the overall identity and purpose. As each grain looks the same, each one possesses its own uniqueness and sense of distinction. This analysis could also apply to the relationship where each moment is seen as its own specific instant in time where something was revealed and something was explored.
My perception of the quote is that any connection we make during our lifetimes is bound to go through many different events until the events test such relationship to the point of ending them , or making them stronger. Unfortunately, like the previous poster said, we all come to an end, and so will the connections we made as living beings on Earth.
I suppose you could say this is true if you think of the thin part in the hourglass as some sort of troublesome time and you think of the wide parts as the good times.'
If you think of things this way, then the quote is saying that all relationships will undergo a hard time that will come between different periods of happiness. I am not sure that this is true. I imagine that all relationships have hard times. But I do not think the number is limited to one. And I fear that relationships often do not get past the narrow parts and go back to being good.
If you are saying all relationships run out of time and are finished at some point, I suppose that is true because we all die sometime.