This is the rule in life. Everybody, is invariably effected by this.
Something is always missing from our lives. Something that completes a circle; that which makes life a bed of roses, as per the adage.
I was wondering on the same thing. There is always a void that cannot be filled. This is my conclusion after my interaction with the small number of people that i come across. Though, some people seem gay all the time, there is hidden incompleteness within them(my opinion).
Perhaps, that makes us treasure those few gifts that are bestowed upon us by Providence. We try to seek happiness with whatever is given to us. We are supposed to think in that manner, in the least.
As for the those things that are always illusive. The more we think about it, the longer the list gets.