Musar על עירובין 43:1

The means that lead a person to this goal are the commandments which the blessed G-d commanded to us. The place of the performance of these commandments is only in this world. Therefore, man was first placed in this world so that through these means prepared for him here, he will be able to reach the place prepared for him, namely, the World to Come, there to be sated with the good which he acquired through these means. This is what our sages of blessed memory said "today to do them, and tomorrow to receive their reward" (Eruvin 22:1).