God alone is. Simply is. By His own infinite exercise of existing.
Everything else that exists, exists because God gives existence.
To understand this is to understand all, the inner truth of everything—the mystery, so to speak, of the cosmos. God has given existence to all these things that are not Him, but exist like He exists, as opposed to not existing—things like the earth, us, trees, the sun and moon, water, etc.—He has given being to all these non-divine things; He has shared being with them, for a reason.
To communicate with us. To draw us to Himself, lift us up to Himself, make us His friends. His intimates.
We worship Him for His awesome majesty. We cannot imagine the power and beneficence that possesses being absolutely and shares it generously. We respond to His love with love.
He Who is is: the infinite communion of eternal love. His Son, meek and humble of heart, revealed the divine love, the tri-unity. Everything that He has made draws us into that communion.