There are several reasons why Pranab Mukherjee has lost the enthusiasm he had for the President's job in 2007.

Diptosh Majumdar in the DNA tells you why:

He has become more immersed in governance and may find a passive stint in Rashtrapati Bhavan simply too boring.

Secondly, he knows that even if there is early speculation about his being in the race, the Congress leadership may finally deny him the opportunity to be a candidate. Mukherjee realises that he is a politician of immense experience and nobody trusts such a person for fear of dealing with a politically active Rashtrapati Bhavan on a day-to-day basis. The bitter experience which former prime minister Rajiv Gandhi had during the tenure of a combative President Giani Zail Singh taught the ruling parties a lesson. They have become averse to nominating influential politicians for the huge assignment.

Thirdly and more importantly, Mukherjee is aware that though he is widely respected across parties and has established a good relationship with a large number of senior politicians including those of regional parties, it may not be easy to put together the required number.