The Briton says he was convinced his team was set to win the championship after its strong start, and saw Lotus as a stronger threat than Red Bull.
McLaren ultimately finished third in the constructors' standings, between Ferrari and Lotus, as Red Bull swept to the title.
"After five races I thought that we would win the championship," said Button, who took victory in the season-opening Australian Grand Prix after a very positive winter of testing.
"We seemed very competitive, and I thought Lotus were going to be extremely good this year. They were consistent but they didn't have the outright pace."
The Briton added: "Five races in I wouldn't have said that the Red Bulls would win the championship this year."
McLaren had a mixed season in 2012, with a very quick car on most circuits but with reliability issues hurting its chances.
Lewis Hamilton secured seven pole positions and four wins, while Button won three races and grabbed one pole.
Despite the pace of their car, Hamilton and Button finished in fourth and fifth places in the standings.
Button conceded McLaren's efforts had simply not been good enough.
"You always think you've lost the title if you haven't won the championship," he said. "We're all fighting for the world championship and only one team can win.
"Red Bull did a very good job and won the constructors' championship, whereas with us, I think we've still done a good job, just not quite good enough.
"But it's a very competitive sport and we shouldn't be disappointed with how we've gone about our racing, we just haven't been good enough this year."
The Briton said Red Bull's consistency and reliability was key in the team securing its third consecutive championship double this year.
"In terms of the team, we weren't very quick before the summer break," he said. "Before Hockenheim we didn't have the pace, but with the update in Hockenheim we moved forward a lot and we carried that through for most of the year.
"But there's one team that has been quicker than us for a lot of the time - and that's Red Bull - and more consistent than us and they've been more reliable as well."