Because it is contained in Scripture, "Behold, I lay in Zion a chief cornerstone, chosen, and precious: He who believes in him will not be disappointed."
Peter clarified two relationships of the believer in these verses (4-6). He rests on Christ as a building rests on its foundation. Furthermore he relates to every other believer as the stones of a building under construction relate to one another. We need each other, should support each other, and should work together to build the church in the world.