"He didn’t enjoy the new engine, the feel from the new system, the power unit, the brake by wire, the lack of downforce. You could tell he wasn’t happy.
"He was preoccupied and to compound that his team mate (Australian Daniel Ricciardo) won three races. There was that feeling 'Am I enjoying this as much as I thought I was?'"
While Ricciardo won three races, Vettel, who had been champion for four years in a row and won 13 grands prix in 2013 including nine consecutively, ended the campaign empty handed.
The German has since left Red Bull and joined Ferrari, who also failed to win a race last year for the first time since 1993.
"It was like someone had taken his toy away. It took him a while to get to grips with that. It was not something he was used to. He went through a period of disillusionment about the direction Formula One was going in," added Horner.
"There was a stage last year when he thought whether he wanted to stop or not, whether he was getting the same level of enjoyment or not and whether or not he wanted to continue."
Horner, whose team finished runners-up to Mercedes in 2014, said Vettel had been so successful in previous years that he struggled to come to terms with a car that was not to his liking.
"It raised some questions he had to deal with. He went back to basics and drove a kart in the middle of the year to get back to the bare essence of why he was a grand prix driver and rediscovered his passion for being a grand prix driver," he said.
Horner said it would take time to get used to seeing Vettel in Ferrari's red overalls but felt the German was right to seek a change and a new stimulus.
"His boyhood hero was Michael (Schumacher) and, of course, there was the lure of Ferrari. For any driver -- the brand, the history, the mystique, is immensely powerful."
(Reporting by Alan Baldwin, editing by Ken Ferris)
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