You are in me

It is said in one of the Upanishads:

..... the supreme soul, is in me, as well as in my son, and my joy in my son is the realization of this truth, It has become quite a commonplace fact, yet it is wonderful to think upon, that the joys and sorrows of our loved ones are joys and sorrows to us-nay, they are more.

Why so?
Because in them we have grown larger, in them we have touched that great truth which comprehends the whole universe