“Let your heart overflow with thankfulness, and I will fill you with my joy”-Sarah Young
The word picture I see here is a cup spilling out because it is too full. 

Normally, when that happens-unless there is a source to keep filling it like a cup under running water- the cup will ultimately either empty or become stagnant at about 2/3 full.

Neither full nor empty. 

A land of apathy per se. Or maybe sitting in the dormancy of a status quo.

The fear could be to not appear TOO thankful- maybe it will be taken from me if I show I like it too much? Maybe there is something better out there so I don’t want to waste my feelings on what might not be the best?

So we say “thank You God” and don’t overflow the cup- instead just let it spill out a bit.

But here, God is saying LET your cup overflow…there is no fear that the cup will empty, that nothing else good will come, that there is nothing better because in this moment this IS THE BEST thing…

Let it overflow AND I WILL FILL YOU WITH MY JOY.

Spill out thankfulness and be filled with joy.

Repeat.

See the cycle???

See the image?

Thank you Lord for my loved ones…I am filled with joy at the ridiculous memories of years gone by…thanks bubbles out again…joy replenishes what has poured out.

Thankfulness OUT, joy IN…sounds like a good deal to me!