"God doesn’t want your penance, or your religious work, or your bulls and sheep and rams: God wants your love. And love, it turns out, is not a fruity emotional cocktail...Setting our affections on God is not the same thing as sending Him a Valentines Card every day."
How quickly do our hearts stray from the affection we are supposed to have for the LORD? Even after God had been faithful to his people...Moses still found it crucial to remind them to remember the LORD:
 “And when the LORD your God brings you into the land that he swore to your fathers, to Abraham, to Isaac, and to Jacob, to give you—with great and good cities that you did not build,  and houses full of all good things that you did not fill, and cisterns that you did not dig, and vineyards and olive trees that you did not plant—and when you eat and are full,  then take care lest you forget the LORD, who brought you out of the land of Egypt, out of the house of slavery.  It is the LORD your God you shall fear. Him you shall serve and by his name you shall swear.  You shall not go after other gods, the gods of the peoples who are around you— for the LORD your God in your midst is a jealous God—lest the anger of the LORD your God be kindled against you, and he destroy you from off the face of the earth. | Deuteronomy 6:10-15 ESV
And Finally, a word from Tim Keller on this subject: