Although God has regularly been conceived of by Christians as an almighty king and judge, he has primarily been depicted as a loving father (a). Of course, this is nothing new to New Testament Christianity. The Old Testament people of God likewise viewed Yahweh as a loving father to His people. Jesus reinforces this image, though he also expands it considerably. Not only is God the father of His people, He is abba ("daddy"). Jesus is signaling to us that God is a loving, tender, compassionate parent who seeks a genuine and intimate relationship with His children.
Some commentators see the verses above as proclaiming the sonship of Jesus. Certainly the Bible teaches us that Jesus is the Son of God. However, this passage isn't about Jesus' relationship to God, but rather the disciples. Jesus was trying to teach Peter and the other disciples a very important lesson about who they were. They were a child of the father-king, and as such should live accordingly. They no longer paid religious taxes (or tithe) out of duty. In fact, they didn't even have to pay it at all. But of course, no true son of the king thinks this way. As the King sacrifices for His people, so should the King's son. Jesus is telling his disciples that they should willingly give, not out of duty, but rather out of delight.
We are truly free. Our father is the king of Kings. There is no law to which we must answer nor a code we must follow. In many respects, we are "above the law". Our father is an absolute sovereign, and we enjoy the freedom granted by being His children.
Because we are free, we do not have to give.
But, because we are a child of the King, we give willingly.
Does this sound like a contradiction? Not at all. Servants give because they must, or because they are trying to impress their master. Loving sons and daughters recognize that our parents love us no matter what. We do not give to earn their approval--no matter what we do God will never love us less. We don't give in order to get God to love us, we give because he loves us.
(a) Davies & Allison, Matthew, vol II (ICC), p 748.