The Bible says "Do not worry" and "Do not be anxious about anything" and "Cast all your cares on him", yet we worry.
Don't we believe that God is able to meet our every need? That in all things God works for the good of those who love him? That in all things we are more than conquerors?
Don't we trust God with our lives?
Is it because of our unbelief? Is it because we know that God's idea of what's good for us does not necessarily include our comfort, and will most likely involve growing pain?
We like to be in control - its feels much less risky that way - so we give God a little part of our lives, the "big" problems, and worry about the rest ourselves.
Numpties, the lot of us!