Asked about New Zealand's relationship with the US, he said: "I think we've gone back too much from where we were.
"We have a stronger voice and more respect internationally by being independent. And I don't think the cost is that great."
He said he had no issue with gay marriage or same-sex adoption, and acknowledged that updating adoption laws - which the Government has said is not a priority - was long overdue.
He would have to see the detail of any bill to legalise gay marriage or same-sex adoption before voting for them but supported them in principle.
But he is less liberal on cannabis.
"I don't think people should go to prison for smoking a joint but I don't support legalising cannabis."
He said there was merit in the Law Commission's recommendation for a mandatory cautioning scheme.
Under the commission's proposal, a cannabis smoker would receive three warnings; the first two would include information on treatment, and on legal and health issues.
The third caution would require a user to attend an intervention session, and a fourth would result in prosecution. Such a scheme would not legalise cannabis, which would remain a Class C drug.
Associate Health Minister Peter Dunne has said the proposal was "highly unlikely" to be adopted.
On alcohol reform, Mr Shearer supported a split purchase age of 18 in bars and 20 in supermarkets and liquor shops.
He said there was some evidence to clamp down on alcohol advertising, but he wanted to research the issue more.
Mr Shearer said he had his political awakening in 1984, when New Zealand's anti-nuclear position was forming under then-Prime Minister David Lange.
In 1981, during the Springbok tour that polarised the nation, he was backpacking in the UK and Africa.
"My father was a rugby referee and saw it as just a sport, while my friends had all been out demonstrating," he said.
"I was sort of anti-tour, but became very passionately anti-tour when I came back to New Zealand."
