In the book of Hosea, we read that "the land is guilty of the vilest adultery in departing from the LORD" (1:2, emphasis mine). I read that and suddenly comprehended faithfulness from a whole new angle.
I am faithful to my wife. I pledged my life to her. We are raising children together. I work hard every day for her. My thoughts turn to her often during the course of a day. She really is my joy and delight. To think of living apart from her is a horrifying thought. I can't imagine putting asunder what God has joined together. The parallel to our relationship with God is obvious.
There is something valuable in considering faithfulness in a marital light (marriage is, after all, God's idea and patterned after His relationship with us). And the great thing is that this Groom is perfect. We've no reason to be upset, annoyed or unfaithful.
So faithfulness means living my moments with an awareness of deep affection and attachment to my loving God, to whom I have pledged my life. It means growing deeper in appreciation, affection and security as the years go by. It means being in constant wonder as the beauty of my Beloved unfolds with each passing day. It means becoming more and more like the Lover of my soul.
Being unfaithful to an earthly spouse is an awful thing, but departing from the LORD is the "vilest adultery."