3 Then follows: Neither have I walked amid great things, nor amid wonderful things that are above me. It is most dangerous to walk amid mean things, and not to linger amid wonderful things. God's utterances are great; He Himself is wonderful in the highest: how then can the psalmist pride himself as on a good work for not walking amid great and wonderful things? It is the addition of the words, which are above me, that shows that the walking is not amid those things which men commonly regard as great and wonderful.
For David, prophet and king as he was, once was humble and despised and unworthy to sit at his father's table; but he found favour with God, he was anointed to be king, he was inspired to prophesy. His kingdom did not make him haughty, he was not moved by hatreds: he loved those that persecuted him, he paid honour to his dead enemies, he spared his incestuous and murderous children. In his capacity of sovereign he was despised, in that of father he was wounded, in that of prophet he was afflicted; yet he did not call for vengeance as a prophet might, nor exact punishment as a father, nor requite insults as a sovereign. And so he did not walk amid things great and wonderful which were above him.
Source: Homilies on the Psalms (New Advent)