Trust in the Lord, wait upon the Lord and take delight in Him

 Psalm 37:3-7 

Trust in the Lord and do good; dwell in the land and enjoy safe pasture. Take delight in the Lord, and he will give you the desires of your heart. Commit your way to the Lord; trust in him and he will do this: He will make your righteous reward shine like the dawn, your vindication like the noonday sun. Be still before the Lord and wait patiently for him; do not fret when people succeed in their ways, when they carry out their wicked schemes.

The Lord has commanded us to do good and never get tired of doing so (Galatians 6:9) and commit everything to Him. We are called to trust in Him. This leads to His reward of Him fulfilling the desires of our hearts. This aligning with the command in Matthew 6:33 of seeking His will and that all the things that we need in life, will be added unto us. 

The other part of this verse deals with being still and waiting upon Him. As humans, it is not an easy thing to be patient and wait. However, it is something we ought to do, this leads to great rewards. Isaiah 40:31 says: "but those who hope in the Lord will renew their strength. They will soar on wings like eagles; they will run and not grow weary, they will walk and not be faint. The Lord will give strength and an advantage of soaring like an eagle when everybody else is unable to."

However, this is not about tit for tat, where you do something only because of what you stand to benefit. When you fully understand God's real love for you, you'll find pleasure in serving Him as a way of appreciating Him, doing good unto others as you remember the grace that's been shown to you. The benefits comes as an add on (not the motivation).