I never really understood the difference between being grateful and being thankful. I often wondered why we have two words expressing the same thing. Not too long ago, I got a great idea: I LOOKED THEM UP! I was really surprised at what I found.
Gratitude is an agreeable emotion, consisting in or accompanied with good will to a benefactor, and a disposition to make a suitable return of benefits or services, or when no return can be made, with a desire to see the benefactor prosperous and happy.
Thanks is the expression of gratitude; an acknowledgment made to express a sense of favor or kindness received.
Did you catch that? Gratitude is the feeling or sentiment excited by kindness. Thanks are the expression of that sentiment.
It is not enough to have a grateful heart. That is a good beginning. However, if we do not express that gratitude with our lips, we have not been thankful, either to God or man. This includes our spouse, our children, our parents, our employers, etc.
The writer of Hebrews call thanks the fruit of our lips:
Hebrews 13:15 Therefore by Him let us continually offer the sacrifice of praise to God, that is, the fruit of our lips, giving thanks to His name.