Large language models (LLMs) can both evaluate and explain text quality; however, most existing evaluators operate as static classifiers and lack the ability to refine their reasoning through interaction. We propose an Iterative Alpha–Beta Learning framework that jointly trains two complementary 8B models: an Alpha (𝛼) classifier that assesses pairwise story engagement, and a Beta (𝛽) generator that produces structured, rubric-guided comparative explanations. The two models co-evolve within a closed feedback loop: 𝛼 provides probabilistic preference signals to guide 𝛽’s Direct Preference Optimization (DPO), while 𝛽’s improved explanations are reintegrated to retrain 𝛼 via a KL-based contrastive objective. This dual optimization enables mutual learning: 𝛼 gains interpretability and robustness from 𝛽’s textual rationales, while 𝛽 acquires stronger alignment and discriminative precision from 𝛼’s confidence deltas. Experiments on human-annotated story-pair datasets HANNA show that the proposed system consistently outperforms strong single-model baselines in both accuracy and explanation quality across multiple iterative rounds.