It may sound strange, or to the super-religious, irreverent, but we find the term and rationale in the middle of scripture: Romans 8:15 So you have not received a spirit that makes you fearful slaves. Instead, you received God's Spirit when he adopted you as his own children. Now we call him, "Abba, Father."
Abba is the Aramaic term children would call their father. It's an intimate expression. It's not a stretch to say that Paul means that in Christ and through the Holy Spirit we can now call God "Daddy."
God wants us to relate to Him as His dear children. It's not a bad litmus test for the intimacy of your relationship with Him: is addressing Him as "Daddy" in prayer completely out of the norm and uncomfortable? Jesus says, '“Truly I tell you," He said, “unless you change and become like little children, you will never enter the kingdom of heaven,"' in Matthew 18:3.
This is not a license to sin nor a reason to think there will not be consequences for disobeying God because you're a favorite of "the Man Upstairs." If you've embraced Jesus as your savior and are following Him, you've been invited into the family of God and into a love more rich and free than you could find anywhere else.