Why is the word 'perfection' associated with the Taj Mahal?

Taj Mahal has an enduring legend and its status as one of the 7 modern wonders of the world makes it undoubtedly a perfection piece.
However, there are key elements which contribute to its perfection status:
Taj Mahal is the apogee of the evolutionary architectural process in medieval
India.
The sublimity of the building comes from its orderly, simple plan and elevation, amazingly perfect proportions or symmetry, the ethereal quality marble has lent to it, the perfect setting of bagh and river and the pure outline of the tomb silhouetted against the sky.
The tomb is laid out in a Chahar Bagh, criss-crossed with paths and water courses, interspersed with pools and fountains.
At the corners of the terrace stand four tall, tapering minarets, one hundred and thirty two feet high. The main body of the building is topped with a drum and dome and four cupolas forming a beautiful skyline. The plinth, the walls of the structure and the drum-dome are in perfect proportion to one another. Towards the west of the white marble-faced tomb lies a red sandstone mosque and a similar construction in the east to maintain balance. Marble for the building was quarried from the Makrana mines in Rajasthan and this white edifice is contrasted with the red sandstone of the surrounding structures.
The tomb structure is a square with chamfers forming eight sides, recessed with deep arches. This structural stylization produces in the elevation of the building a variety of contrasting planes and shade and solids and voids effects.
Light to every part of the building is obtained by means of carved and perforated jalis, set in the arched recesses of the interior.
 Lastly, the art of calligraphy is used with the inlay of jasper in white marble to write Quranic verses. Calligraphy provided a decorative element on the walls and a continuous connection with the Almighty.