Cockerill rightly describes how scandalous this was (Hebrews, 137):
The pastor boldly asserts his shocking thesis: it was “appropriate” to the character and purposes of the sovereign God, the source, judge, and goal of “all,” to use suffering to equip the Savior so that God could fulfill his purpose for his “sons and daughters.” Christ’s suffering was neither a logical necessity forced upon God nor a mere decision of his will, but an appropriate expression of the divine character. Thus his incarnate suffering was integral to the Son’s person as the ultimate revelation of God’s nature. The pastor would persuade his hearers by enabling them to see and feel the way in which the suffering of the Savior was most appropriate to God’s character in light of their need.
Kleing, John Concordia Commentary: Hebrews footnote 30, 132