"One thing is clear, our campaign did not click. Our message did not reach the people. Particularly, the first-time voters wanted change. Even in politics, there is an 'expiry date' and a shelf life. People wanted change, and there was a big wave for change," Kamal Nath said in a show on India TV news channel.
He said that even if Narendra Modi had not been there, the Congress' tally would have declined.
Asked whether Priyanka Gandhi Vadra should join politics, he said it was up to her and her family to decide when she should join politics.
Asked whether party leaders were trying to prevent Priyanka Gandhi from joining politics, he said: "We are not preventing. Priyanka Gandhi ji must decide herself. She would have to decide in which manner she wants to join politics. Her family will have to decide. No doubt, we can ask her, and we may like her, but I can't decide on behalf of her and her family."
On the factors behind the recent electoral debacle, Kamal Nath said: "There is no single reason behind such a huge defeat. Every state had its own reason. Our party is going to introspect, as we have to contest more elections."
The Congress leader said he had a long meeting recently with party vice president Rahul Gandhi.
"I had a long meeting with Rahul ji. He is eager to carry out introspection, because after all, every political party should act as a guardian of people's interests. As an opposition, we want to act as a guardian for those whose voices are not heard."
Kamal Nath hit out at the Bharatiya Janata Party for opposing talks with Pakistan when the party was in the opposition, and then inviting Pakistani Prime Minister Nawaz Sharif as soon as it came to power.
"The very same BJP which used to demand 'no talks with Pakistan' over the beheading of our jawans, invited their prime minister as soon as it came to power. They changed their stance on the first day itself.
"They (BJP) had even objected to our prime minister speaking to their prime minister over phone three years ago. Today, the same leaders are inviting them to dinner, and hugging them, and exchanging shawls."
Kamal Nath also raised the issue of Human Resource Development Minister Smriti Irani's educational qualifications.
Irani is at the centre of a controversy that she had misrepresented facts about her educational qualifications in her election affidavits.