Isa is like a creative volcano, ready to explode at any moment with his love for Jahanara. Because he can't spew his love-lava all over Hindustan, he builds the Taj Mahal as an architectural symbol of his love:
"Do you know, Jahanara, what I think of when I design? I think of you. I hold your face in my mine and seek to mimic its loveliness. I remember the shape of your body and try to equal its brilliance."
"You do?" I asked, immensely surprised.
"I watch how the sun reflects off your cheek, and I build so that the sun will dance off the marble in the same manner. I survive your absence in my heart, not having you as the mother of my children, by shaping stone in your image."
"Not my mother's?"
"No," he whispered, then sighed. "I can't share my love with you as I'm supposed to, the way a man shares such love with his wife. And so I build. I build to honor you, because this is the only way that I can love you, by sharing my love with the world. The first stone I laid had your name chiseled into its underside and the last—please grant me this wish, Allah—shall carry both our names." (8.54-58)
A cornerstone is literally what it says it is: it's a stone on the base or corner of a building. Sometimes you'll even see plaques on the cornerstones stating the builder, the date the thing was built, and maybe some information about the building's historical significance. However, originally, the cornerstone was the first stone laid in a masonry foundation; it was crucial because all other stones were set in reference that that one.
For Isa, Jahanara is not just the literal cornerstone—the stone with her name written on the bottom of it that is the reference to all the rest of the Taj Mahal—she's also his figurative cornerstone, as well. His love for her forms the basis of everything he thinks and does. All of his gestures touch back on his love for her, and all of his actions will be in reference to his love.
Excuse us while we break out the tissues…