There is a certain type of person, one who most of us will or have met, one who comes into our world to cause a change, and one who will leave once that happens.
The truly difficult and sad part is when you constantly find yourself being the passer-by, the never-here-for-long, the just-on-the-way, and you get used to it.
You start to see people with expiration dates on them, relationships become somewhat meaningless, but you still cultivate more of them; especially because you know they will not require nurturing and can just be cast aside when the time comes - time which you have already accepted will be sooner rather than later.
It is very sad to think of this as the new normal.