Isn't Aiming For Rewards Selfish?

First of all, God created us to respond to rewards. That's our nature. He created us with certain legitimate desires. The rewards address and satisfy these desires.

Secondly, the answer to this is the proper understanding between healthy self-interest and selfishness.

Selfishness is the attitude that thinks of oneself only and the pursuit of gain at the expense of others. It is different from healthy self-interest. Healthy concern for oneself is normal. We eat, we drink, we take care of our bodies etc. We can be concerned about ourselves while, at the same time, aim for the glory of God and pursue the good of others. Everybody wins --- God is glorified, others are served and ourselves rewarded.

A selfish person, on the other hand, would pursue his own gain and interests even at the expense of God's glory and the good of others. In this case, only self wins.

Thirdly, the right-hearted Christian will sublimate His rewards to glorify even more the King of glory:

Re 4:10 "...and cast their crowns before the throne..."

What is the "joy of the Lord" in Nehemiah 8:10?

 Ne 8:10 Then he said to them, "Go your way, eat the fat, drink the sweet, and send portions to those for whom nothing is prepared; for this day is holy to our LORD. Do not sorrow, for the joy of the LORD is your strength."

Just move down to verse 12 and you will see the parallel:

Ne 8:12 And all the people went their way to eat and drink, to send portions and rejoice greatly, because they understood the words that were declared to them.

It refers to the joy of understanding God's will. Joy is a great encourager. When you delight in God's will you will do it.