The current manager is set to leave Celtic in the summer.
The Norwegian boss has confirmed he will leave the Hoops at the end of the season after two years in charge.
It has been two difficult years for him, as his side have failed to qualify for the Champions League and lose out in the cup competitions.
Last season they were beaten by Inverness in the Scottish Cup semi-final, while this year has seen defeats at the same stage to Rangers, and an exit to Ross County in the Scottish League Cup.
He has managed to secure back-to-back titles, but as far as Deila is concerned there is nothing he could have done that would have been good enough.
“Celtic had won everything before I came and everything you do will never be good enough, almost," he told the Telegraph.
“The challenge has been unbelievably big and all these experiences have done something to me as a manager and person. There are things I have done very well and things I could have done better.”
The major problem for Deila was that he arrived following an extremely successful spell under Neil Lennon.
His sides were particularly strong in the Champions League as they made it into the last 16 and beat Barcelona, something that is sill fondly remembered.
That left Deila with a very high watermark to reach, something he failed to do, and perhaps his comments are not far wrong.