The spirit of the NPT creates a divide, between countries that did develop nuclear power before 1967 and those that didn't develop nuclear power before 1967. It only gives the 'Permanent 5' the right to hold weapons. Although it permits the use of nuclear energy for constructive purposes, it puts all the other nations at risk.
India, despite being a nuclear weapons state, would have had to sign the treaty as a non nuclear weapons state, and in addition has to undergo inspections. [2] The NPT, in India's opinion doesn't explain the need for this distinction and loss of national sovreignity.
India did however; sign the "no first use" treaty (which later changed to the no first use against non-nuclear weapon states) in 2010. [3]