ʿabd–rabb
The servant–Lord relation: the creature as ʿabd (servant, worshipper) before God as rabb (Lord, sustainer). In the Māturīdī sources this is the frame divine love actually lives in — reverent obedience upward, approval and protection downward — rather than a fellowship of equals.
Arabic ʿabd, “servant”, and rabb, “Lord”.
The ʿabd–rabb relation names the two poles of the creature before God: ʿabd, the servant or worshipper, and rabb, the Lord who sustains. In the Māturīdī sources this is the frame within which divine-human love is actually described — not a fellowship of equals.
What runs each way is asymmetric by design. Upward: reverent obedience and self-giving action. Downward: approval, praise, acceptance, favour, protection, reward. The corpus's adjudication grants that this is a real, morally discriminating relation — more than bare capacity on either side — while noting it is not eternally actual interpersonal communion between equals.
Why it matters to the why-three question: the EO comparison is not "love versus no love" but "communion of equals versus Lord-and-servant." Whether reciprocal communion is a necessary perfection that a Lord-and-servant frame cannot supply is exactly what still has to be argued.
The mistaken everyday reading is "slavish submission and nothing else." The sources describe a relation with real moral texture in both directions; the dispute is about whether that texture is enough.