The first “not good” thing in the Bible is Adam’s isolation and aloneness (Genesis 2:18). It’s the one discordant “not good” in a string of “goods” that ends with a “very good.”
Later, Jethro tells Moses that spending his day judging the people without any help is “not good” (Exodus 18:17). Moses needs “helpers suitable to him” in the judicial sphere, as Adam needed a liturgical partner in the garden sanctuary.
Isaiah implies the same thing (65:2). Yahweh stretches out His hands in pleading and petition to a rebellious son (cf. Deuteronomy 21:18, 20). He is the father of the prodigal waiting at the door to welcome home his wayward son. In their rebellion, they “walk in the way that is not good, following their own thoughts.”
What’s not good is that Israel walks alone, impervious to correction from God or man. No one speaks, no one rebukes, no one tries to redirect. They have become dull of hearing, and without open ears they are alone on their dangerous path. They are perfectly content, alone with their own thoughts.
Political, moral, marital goodness requires otherness and relation. It is not good to be alone.