We humans spend a remarkable amount of time complaining (at least internally, if not verbally) about things that we're actually grateful for. With me, it's usually, "This job is boring, I'm tired, I don't want to be here." I get so caught up in one moment of how I feel that I ignore what a blessing it is to have a job at all, and especially a fun, convenient, flexible job with great coworkers.
Out of the abundance of the heart, the mouth speaks. Maybe our grateful speech outweighs the ungrateful, but all that leaves us with is a big jumble of hypocrisy. We can sing the Doxology in church, but if we spend the other six days of the week griping, our "Praise God from Whom all blessings flow" isn't doing us much good.
James gives complaints and ingratitude no quarter, no matter who we're complaining about: "With the tongue we praise our Lord and Father, and with it we curse men, who have been made in God's likeness. Out of the same mouth come praise and cursing. My brothers, this should not be." One person speaking both curses and praise--true praise--is like a spring that gives both fresh and salty water: impossible. It's one or the other, all or nothing.
I will bless the Lord at all times; His praise shall continually be in my mouth.