Willingness. Rather, I put it as the motive why we serve Him.
I used to think why do we obey our parents? Because we want to be the good boys and girls in people's eyes, because we want some rewards, because we want them to love us. Well but at the end, it's just simply because they are very much right all the time because they've been through thick and thins in life and simply because it's the right thing to do and we love them. In fact, they loved us first, even before we understood the meaning of love. Sacrificial, unconditional and unrequited. However, their love comes with a limit.
But our Father in heaven, He is a God of no boundaries. He doesn't just love us but instead, He is a God of grace. A God who is willing to pick us up each and every time we fall, a God who is willing to clean up our mess when we get so dirty again and again, he is so patient with us. So patient even sometimes I think He gave me way too much than I deserve.
We used to study science about a little bit of astronomy. Then it was said that there are billions of stars in the sky. Then it dawned on me that God actually cared for single one of us on this earth, these miniature humans compared to the billions and billions of stars. What are we, that He should even notice or say even listen to our little meaningless complaints or unreasonable requirements. This love of God is somehow, incomprehensible.
Similarly, why then do we serve God?It's not supposed to be just to receive the assurance after we serve. It's not the appreciation of others. But instead It's because we want to show Him that we love Him. It's because He deserves us to serve Him. Human's assurances and affirmations will never satisfy our thirst. It's God's assurance and affirmation we ought to seek and chase after. He has given us more than we human beings, these miniatures in this gigantic universe deserve. He is so patient with us even when we simply don't and never choose to learn from our lessons, so tolerant when we get tired and weary and worn, so loving even when we are so unlovable, self-centered and stubborn.
Serve because we can never repay Him. Serve because we know we can't never give God our entire being as selfish Homo sapiens but He deserves our time, our sweat and our entire life and after life. Serve because we love Him, and because He deserves our love. You'll never know, perhaps He's preparing a grand banquet in heaven waiting for you, saying "You are one faithful servant and I have found favor in you." :)