"You need to grab those opportunities, those moments, because you can't get them back," he tells PEOPLE

By Peter Mikelbank
Updated November 18, 2015 01:20 PM
Valery Hache/AFP/Getty

As Monaco prepares to celebrate National Day on Thursday, Prince Albert is reflecting on his 10-year reign and his family life with wife Princess Charlene and their 11-month-old twins, Prince Jacques and Princess Gabriella.

“My happiest [achievements] have to be my marriage and my children,” he tells PEOPLE in an exclusive new interview. “That stands above the rest. But I’m happy about some of the changes we’ve made in Monaco, the way Monaco is progressing. And I’m happy about the small part my foundation has played for the environment.

“There’s more consideration concerning the oceans now. There’ll be an Oceans Day at COP21 (the United Nations Climate Change Conference) and people are generally becoming aware of the role they play.”

On Thursday, the 57-year-old royal will mark National Day alongside Charlene and the twins. Earlier this week, he spoke to PEOPLE about the Paris terrorist attacks in neighboring France, noting that tragedy will cast “a more somber mood” on the day’s events.

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When it comes to the everyday business of governing a nation, he says the biggest challenge is what he calls “the people problem.”

“I kind of knew it before, when my father reigned, but it is spending time with people who don’t want to work as a team,” he says. “So much time is wasted in trying to convince people to go in the right direction. It is the same for any head of government or corporation: You spend so much time in getting people to all row in the same direction.”

Putting the interview on hold for a moment, he returns and says with a laugh, “Sorry! It was a Daddy thing.”

“Actually that’s been the challenge of the past year, finding enough private time to be with my wife and kids.”

Fatherhood “changes your outlook,” he adds. “You learn to think more ahead. I kind of knew I would be evolving. Other fathers told me that, and it’s true. You have to take your time, take advantage of every moment you can because it goes by so quickly.

“It has gone by very fast, in fact. And you need to grab those opportunities, those moments, because you can’t get them back.”