The Anfield star said: "I have to constantly prove myself. You have to prove yourself in every game for England and Liverpool, two massive stages to play on.
"If your form dips a touch, you are replaced. You've got to keep on your toes otherwise there will be someone there to replace you. There is always someone to take your place."
Rafa Benitez has released Anfield legend Robbie Fowler but is determined to increase competition for places up front this summer.
Crouch added: "When you've got 70-odd games in a season, it is not feasible to start every game. The manager knows that but players are greedy. They want to play every game.
"I'm no different but I understand that you can't."