In a wide ranging 'Beyond the Grid' podcast with Formula 1's Tom Clarkson, Horner shed some light on the reasons why, in his view, the seven-time Grand Prix winner decided to leave the Milton Keynes outfit for Renault.
"I guess in his mind he’s felt that after five years he feels he needs a new challenge, a new prospectus" said Horner.
"I think that obviously he’s chosen to join the Renault team, that he feels they’re in the ascendancy.
"He knows the Renault product very well from all the time that he’s spent with us. And I can’t help but feel that he wants to be a leading role perhaps in a smaller environment.
"The competition between he and Max is very intense. Max is growing stronger and stronger. And I think Daniel’s just decided that the timing is right for him to check out and try something else."
"He sees Max growing and growing in terms of speed and strength and he doesn’t want to play a support role, I guess, for want of better words," added Horner.
"Not that they’re in any way treated in any way different. I can’t but feel that was perhaps a large part of Daniel’s decision-making."
"I could understand if it was to Ferrari or Mercedes. But it’s an enormous risk at his stage in his career."
From offering Ricciardo the guarantee of an equal status to complying with the driver's financial demands, Horner insisted that every effort had been made to keep the Honey badger on-board.
Ultimately, it just wasn't enough however. And the Red Bull Racing boss offered aa perfect analogy.
"It's been a bit like trying to convince a girl to go out with you, but she’s being pretty reticent, he said.
"We have bent over backwards to make it happen, but if someone’s heart isn’t really in it, then…
"We gave Daniel everything he wanted and asked for and it still wasn’t enough.
"We were even prepared to do a one-year agreement so he was available to Ferrari or Mercedes should they come knocking in 12 months’ time.
"It wasn’t about money or status, commitment or duration. I think he felt 'I need to take something else on in this stage of my career'. It might be an inspired choice, it might be one that he regrets."