He learned obedience

...when we speak of the death of our Lord upon the cross as the supreme act of his obedience we are thinking not merely of the overt act of his obedience we are thinking not merely of the overt act of dying upon the tree but also of the disposition, will and determinate volition which lay back of the overt act. 

And furthermore, we are required to as the question: whence did out Lord derive the disposition and holy determination to give up his life in death as the supreme act of self-sacrifice and obedience? 

We are compelled to ask this question because it was in human nature that he rendered this obedience and gave up his life in death. And these texts in the epistle to the Hebrews confirm not only the propriety but the necessity of this question. For in these texts we are distinctly informed that he learned obedience, and he learned this obedience from the things that he suffered. 

It was requisite that he should have been made perfect through sufferings and become the author of salvation through this perfecting. It was not, of course, a perfecting that required the sanctification from sin to holiness. He was always holy, harmless, undefiled, and separate from sinners. But there was a perfecting of development and growth in the course and path of his obedience - he learned obedience. The heart and mind and will of our Lord had been moulded - shall we not say forged?- in the furnace of temptation and suffering. 

And it was in virtue of what he had learned in that experience of temptation and suffering that he was able, at the climactic point fixed by the arrangements of infallible wisdom and everlasting love, to be obedient unto death, even the death of the cross. it was only as having learned obedience in the path of inerrant and sinless discharge of the Father’s will that his heart and mind and will were framed to the point of being able freely and voluntarily to yield up his life in death upon the accursed tree.  

[Redemption Accomplished and Applied , John Murray p 22]