Hadith.2114 - The House (Ka'bah) was placed in the center of the Earth because it is the spot from beneath which the Earth was spread out, and so that its obligation could be equal for the people of both the East and the West.
The Black Stone (Hajar al-Aswad) is kissed and also touched to fulfill the covenant taken with Allah سبحانه وتعالى, the Almighty, in the primordial covenant.
Allah سبحانه وتعالى, the Blessed and Exalted, placed the Black Stone in its current corner because He took the covenant from humanity in that specific location.
The tradition of glorifying Allah سبحانه وتعالى through takbeer (saying "Allahu Akbar") and facing the corner containing the Black Stone from the hill of Safa began because when Adam عليه السلام looked from Safa and saw the Black Stone placed in the corner of the Ka'bah, he glorified and magnified Allah سبحانه وتعالى with takbeer, tahleel (saying "La ilaha illa Allah سبحانه وتعالى"), and tamjeed (praising).
The covenant was placed in the Black Stone because when Allah سبحانه وتعالى, the Exalted, took the covenant affirming His Lord عز وجلship, the Prophethood of Muhammad (peace and blessings be upon him and his family), and the leadership (wasiyyah) of Ali عليه السلام, the angels trembled in awe. The first to hasten to acknowledge and affirm this was the Black Stone. Thus, Allah سبحانه وتعالى chose it, entrusted it with the covenant, and placed it as a witness. On the Day of Judgment, it will have a tongue that speaks and eyes that see, testifying on behalf of all who honored the covenant at that sacred place.
The Black Stone was brought down from Paradise to remind Adam عليه السلام of the covenant and pledge he had forgotten.
The boundaries of the sanctuary (haram) are as they are because Allah سبحانه وتعالى sent down a red ruby for Adam عليه السلام, which was placed where the Ka'bah now stands. Adam would circumambulate it, and its light extended to the marked boundaries. The landmarks were established based on this illumination, designating the extent of the sanctuary.
The Black Stone is touched because it holds the pledges of all creation. It was originally brighter than milk but became blackened due to the sins of humanity. Were it not for the impurities of ignorance that touched it, no afflicted person would touch it without being healed.