Jesus was not a super-hero.

Jesus was not a super-hero.  Jesus was a man.  Jesus was also Emmanuel, God with us.  Jesus is the god-man.   Philippians 2 describes how he did not cling to his divinity, but limited himself in humanity.  He then grew and learned and had real choices.  Jesus was more like us in every way except sin.  His way of dealing with life is totally human.  It is not 'other' than ours.  It is the way things should be.

For it was fitting that he, for whom and by whom all things exist, in bringing many sons to glory, should make the founder of their salvation perfect through suffering.  (Hebrews 2:10)
Since therefore the children share in flesh and blood, he himself likewise partook of the same things, that through death he might destroy the one who has the power of death, that is, the devil,  (Hebrews 2:14)
Therefore he had to be made like his brothers in every respect, so that he might become a merciful and faithful high priest in the service of God, to make propitiation for the sins of the people. For because he himself has suffered when tempted, he is able to help those who are being tempted.
(Hebrews 2:17-18)
For we do not have a high priest who is unable to sympathize with our weaknesses, but one who in every respect has been tempted as we are, yet without sin.
(Hebrews 4:15)
Although he was a son, he learned obedience through what he suffered.
(Hebrews 5:8)

Growing in obedience to God as King is painful in a world of rebellion.  The current of this world flows toward compromise.  Jesus faced real temptations, and it really was hard for him.  Yet he did what he was called to do.  He therefore is able to be merciful to us, because he has first hand experience.  He is our merciful high priest.  We are safe from condemnation.  Jesus paid for our imperfection, our immaturity, or disobedience, our compromise, our rationalizing, or foolishness, or sin.  He said it was perfected; finished; telos'ed.  We are not trying to perform for acceptance.  We are accepted.  

What is it that we are doing, then, when we take the message of the sermon on the mount, with it's central point of being 'perfected'?  Why is it so important to grow up?

Growing up to exhibit more of the character of the God who created you to flourish is really good; it is blessed.  God is holy, wise, just, compassionate, merciful and every meaningful moral virtue imaginable.  Growing up, being perfected, is being better and better at giving and receiving love appropriately.  It is being better at dealing with justice and mercy.  It is blessed.  It is right.