Addressing a rally here, Modi said that at that time the Bharatiya Janata Party was not sure why the Janata Dal-United broke the partnership.
"Now we know why he (Nitish Kumar) broke the alliance. The dream of becoming prime minister was not letting him sleep," Modi said.
He also took a dig at the Third Front saying it has a dozen prime ministerial aspirants.
"Look at the Third Front, almost all parties have saved the Congress and supported it at some point of time. There are either former prime ministers, or prime minister aspirants," Modi said.
"There are more than a dozen people who have got clothes stitched to become the prime minister," he added.