Moses told the Jewish People, "Behold I have set before you today life and good and death and evil. Choose life!"
Devarim 30:15-19
It is not always clear that good behavior leads to blessings and life and that bad behavior leads to curses and death.
This allows us the free will to choose to be good.
If it were always clear that good behavior leads to blessings and life, whereas bad behavior leads to the opposite, what choice could we have but to be good?
The very fact that being good does not always lead to goodness both forces us and enables us to base our relationship with G-d on a more profound basis.
For this reason, G-d (through Moses) is here asking us to be good for its own sake, rather than for any expectation of material reward, even when we do see clearly that being good leads to good results.
From Kehot's
Daily Wisdom #1