God's behavior is rooted in His identity.
God is faithful, so He behaves faithfully towards me. His faithfulness is not contingent upon me but upon Himself. No matter what I choose to do or not do He remains faithful. My unfaithfulness will not make Him unfaithful. He cannot be unfaithful because He is truth, and, thus, always true to Himself and His word.
God reveals His faithfulness to me because He is kind. He is kind because He is gracious. He is gracious because He is merciful. He is merciful because He is good. He is good because He is love.
And He loves me. He loves me because He created me, and He created me because He is God. He shows His love to me through His faithfulness. And He is faithful because of His character.
Consider the following statement I read from one of my daily devotionals: "Having faith in God is not just thinking you know what you believe. It’s being sure of who God is." (Author unknown.)
I have faith in God because He proves Himself over and over again to be constant--never changing--faithful in all of His attributes.
God is faithful because He is faithful God.