The Sages of the Talmud (Brachos 33b) say “Everything is in the hands of Heaven, except fear of Heaven.” That is to say, that although it may appear we have control over our activities, the results and our success are actually beyond our control. However, G-d did leave one matter in our control: the choice to fear G-d, to acknowledge that He is always present, and to act in a matter befitting that awareness.
Since the choice to fear G-d is our own, not in G-d’s hands, as it were, that choice is very dear to Him. It is the only thing he desires from us. “What does G-d want from you? Only to fear Him… (Deut. 10:12)”
Many people, especially the very wealthy, have a fire-proof safe or vault that holds their most valuable, precious items. These might be fine jewelry, personal momentos, or important documents, but all the items in every such safe have one thing in common: they are all precious to the owner, they are things the owner would never want to lose.
G-d also has a vault. “Fear of G-d is His treasury (Isaiah 33:6).” Whenever someone recognizes G-d’s presence, does an act, like a Mitzvah, that demonstrates his awareness of G-d, or abstains from something he knows G-d doesn’t like, that is priceless to G-d. It is irreplaceable. To protect and preserve those good choices, G-d stores them in His treasury, insuring that they will never be lost.
When G-d wants to shower us with blessing, He opens up the vault, as it says, “G-d will open up His treasury of goodness..(Deut 28:12). All the effort we invested to increase our awareness of G-d, all the goodness we gifted to G-d that he lovingly stored away, will be open for us to benefit from its blessings. (Thoughts shared by HaRav Elimelech Biderman, based on Daas Zekainim, Kli Yakar, and Rabbeinu Bachya.)