I'm not a Buddhist; but I do certainly believe in some of its tenets.  Think of the wisdom of this advice from a zen koan

." If you own a teacup that is very precious to you, you have two choices: you can be obsessively careful with it, and live in fear that you'll drop it, or someone will chip it, or an earthquake will come and it will fall out of the cabinet. This object, intended to bring you pleasure, can become a burden.

Or, you can imagine that it is already broken -- because in an important sense, it is. It's sure to break someday, just as you're sure to die and the universe is sure to come to an end. Then, every time you drink from the cup will be a pleasure, a gift from the gods, a special reunion between you and something you had lost. You will be sure to appreciate every chance you have to use it, but having already said goodbye you will not need to use it with fear." (1)

  Or as Audie McCall, a twelve-year Buddhist states: 

  In a nutshell: you can't make someone, even yourself "one", any more     than you can make yourself obey the law of gravity.

You are one with everything, whether you like it or not, and that oneness necessitates inevitabilities and responsibilities that all of us, as human beings, ignore at our peril and the greater suffering of everyone.

For instance, some examples of the inevitabilities: you will die, I will die, everyone who will ever read this will die. Death is inevitable. Pretty simple stuff so far, right?

Here's another inevitability, directly related to the first: you will suffer, I will suffer, everyone who reads this will suffer. 

And another: what you do, and don't do, reverberates throughout existence. You can't avoid this through hiding. You can't wall off your section of existence and hope that all the awful and inevitable things happening on the other side won't creep across.

They will. Why? Because any wall you build is an illusion. Why? Because you are one with everything, whether you asked for it or not, whether you like it or not. (2)

In a now-deleted writeup, zgirll pointed out that you can effectively free yourself by giving the teacup away. This is an asymmetry between the possession issue and the relationship issue: giving away an object is an acceptable way to keep it.

Giving away a person is stupid, unless your relationship (or the person) is dying on its own. The difference is that a possession is something you can fully know, and so your internal model of it can provide the same satisfaction as it can itself.

 Friends and lovers, on the other hand, are far deeper and we never really "figure them out." Ending a relationship that might otherwise have grown is a serious sacrifice which, I think, does not do any good in and of itself and can have serious consequences.(1)

(1) https://everything2.com/title/drink+from+the+cup+as+if+it%2527s+already+broken

(2)  https://everything2.com/title/Make+me+one+with+everything