The Taj Mahal was built for Mumtaz Mahal. It's great vastness is significant to the idea of a life in heaven. This burial site was so important to the Shan Jahan because of their belief of moving into a life after death and he wanted to construct heaven on earth for her. This is shown through the expensive materials like stones and the amount of time it took for this to be built. The garden of Eden in the old testament was used as an inspiration to the building of the Taj Mahal because of their faith in heaven. He wanted her to be in paradise and represented this by building her tomb to represent the islamic Garden of Paradise. Symmetry is used to show that the Islamic Garden of Paradise is perfection. The light colors of stone make the building look weightless despite the weight of the marble. He wanted it to appear that it was rising up and floating above to represent a higher spiritual place.
Sources: A Global History of Architecture, Ching Jarzombek Prakash; http://www.pbs.org/thestoryofindia/gallery/photos/20.html