"But it's important that we don't get carried away and let those incredible achievements affect us, or make us complacent," he adds, earnestly. "We have to try to build on that, which hopefully we'll do with our new album, Midnight Memories, and our film, This Is Us."
Tomlinson's sincerity and modesty, despite One Direction's life-changing stardom, is refreshing. There's no hint of egotism or cynicism, just quiet assuredness and a healthy determination to succeed. Perhaps it's because he appreciates how different their lives could've been - and how lucky they are - after One Direction was created, by chance, when Pussycat Doll's Nicole Sherzinger - a judge on 2010's X Factor - suggested the five of them team-up after they'd all individually flopped during the first round of auditions, for soloists.
"We were so lucky to be put in a band," Tomlinson agrees, nodding. "We certainly weren't expecting to be voted out that quickly! So, when we didn't get through and they offered us the opportunity to put together a band, we all jumped at the chance because we realised we'd been handed a lifeline."
That lifeline has turned into a goldmine, with Up All Night and Take Me Home becoming the third and fourth best-selling albums of last year, with combined sales of 10 million. It's made Tomlinson and his colleagues multi-millionaires, and helped them establish a business empire reportedly worth US$50 million.
Their rise is set to continue with the release of One Direction's third album, Midnight Memories, in November and their biopic, This Is Us. It's a film which charts their ascent to superstardom, with an intimate, year-long look at their lives on the road during their recent world tour. For fans, it's a must-see mix of live footage, fly-on-the-wall reportage and vignettes of their lives, pre- and post-X Factor.
"Having the cameras follow us around every minute of the day was pretty scary, but it shows what it's really like to be in One Direction - and that it's not all just fun," reveals Harry Styles, trying to downplay his hell-raiser notoriety and succession of glamorous girlfriends, including Taylor Swift.
"Yeah, it can be really tough being on the road, touring," adds Tomlinson. "Luckily, we've got each other for support and company, which helps. It must be so lonely being a solo artist.
"You know, even if you take everything out of the equation, like fame or success, we're very lucky because basically we were five strangers when we met and now we're like best friends, or brothers," he concludes cheerfully. "It's great having that chemistry, and I think it shows in the film."
It does. It also shows that despite their dizzying ascent and ensuing fame, they're still as down-to-earth as when they first auditioned for X Factor. It's just as well, because with their third album due out in November - and judging from their past success - the future, from here, is only heading in one direction: up.
This Is Us opens on Thursday.
