Why was the Taj Mahal built?

Shah Jahan promised his queen Mumtaz Mahal when she was dying that he will buils a monument for her that no one else in entire world has built. And therefore he built that monument where graves of mumtaz mahal and later on shahjahans grave lie in place he named Taj Mahal.

An immense mausoleum of white marble, built in Agra between 1631 and 1648 by order of the Mughal emperor Shah Jahan in memory of his favourite wife, the Taj Mahal is the jewel of Muslim art in India and one of the universally admired masterpieces of the world's heritage.