First and foremost, God was literally Jesus' father. One must be obedient and submit to his/her father, especially in Jesus' case. Nonetheless, wisdom is gained through submission.
Secondly, we have this "Great Commission" that is often not taken seriously enough. If we are to be "fishers of men" and truly instruments of the Lord, then how will a lack of submission to our contemporaries give what we say any credibility? In other words, how should be expect the lost to listen to us if we have not found favor with them. The Lord Jesus Christ found favor among God and men and was a GREAT minister to men. Credence was given to his every word, not only because his words were divine in nature, but because people liked him.
How is our relationship with people vital to our spiritual growth? How is it not?!! We grow spiritually and gain a greater appreciation for our Lord when we witness to others. Also, we slowly but surely become a reflection of our the people that we come into contact with on a regular basis. Surrounding oneself with other Christians is a great way of establishing a spiritual accountability. I know that having Christian friends and influences has been instrumentally in my spiritual walk.