The 29-year-old said: "Ask any player and they always want to play.
"But it wasn't to be at Spurs and I'm not a person who likes to sit on the bench and get my money.
"I like to be involved and I love playing. I didn't like to wake up on a Saturday morning knowing I wasn't out on the pitch. At 3pm you're sitting on the bench and it's very difficult to get yourself up for a game.
"I just thrive on playing games and that's why I'm at Celtic. I was delighted to get the 90 minutes under my belt against Kilmarnock. I really needed that and the last 20 minutes I was a bit tired to be honest.
"But I'm a person who gets better with games.
"The more games I get under my belt the more you'll see a sharper and fitter Robbie Keane. I'm not here to prove anything to anyone.
"But I know what I'm capable of doing and I can enjoy my football at Celtic, get my head down and see what happens next season."