The one pierced, mourned as an only son, is finally recognized as your own self. The word behind only son means darling, the one loved as unity, and the whole scene turns on that recognition: the sufferer you looked upon as another was you all along. This waking is poured out as grace, unearned, on the inner state ready to receive it.
The house of David and the inhabitants of the inner city are the states within you into which this dawning comes. You mourn because you see at last that the God you thought you crucified out there, the power you pierced by placing it outside yourself and pleading to it, was your own I AM. The bitterness is the shock of that discovery, and it passes into the deepest recognition a person can have: the Father sought so long is the very self doing the seeking.