Antigonus of Socho, the disciple of Shimon the Righteous, used to say that
a person must not perform the mitzvos only to receive a reward,
Antignos of Socho received (the Tradition) from Shimon Hatzadik. He was in the habit of saying: "Do not be like servants who serve their master in order to get a reward, but be like servants who serve their masters even without payment." The servant in the picture, is waiting on his master and hoping to get paid for his work. When we serve Hashem, we should do so because we love Him. We should not expect Hashem to repay us.