Why can't you just be thankful that you were given life? No matter the circumstance there is always something to be thankful for.
I think this is where you are getting confused. There are always things to be happy
for, sure. I'm very happy I exist. But your Biblical quote makes clear that my life was not made for MY benefit, but for your god's - and that is why "thankfulness" becomes a nonsense.
As for why should you seek Him? ..... Your Heavenly has given you much. Your life, the sun, the stars, the food, those fish even. He didn't walk out. He gave everything for you.. even His Son tasted our death.
Again, NOT given for ME. Everything was created for HIM, you said, digging out a verse in order to prove a point you were making that this world isn't ours, that it was all created for god, not us. But because you didn't think it through - because you've never thought your theology through - you are now desperately backpedalling to say "well, actually, be grateful because it IS all made for you".
But YOU SAID.....
One more thing Earth was not made for us...
If Earth was not made for us, there is no reason at all to be thankful to its creator. But you can't retract, because then you'd have to deny the scripture that you dug out. So you are stuck with the impossible to reconcile "be thankful for something that was created for someone else".