Ammonia (NH₃) absorption drives LiBH₄· xNH₃ through a re-entrant ``solid–liquid–solid'' transition: LiBH₄·NH₃ is a well-defined solid ammoniate, compositions near LiBH₄· 2NH₃ are liquid-like or partially liquefied, whereas LiBH₄· 3NH₃ returns to a more rigid non-liquid ammoniate state. However, the microscopic origin of this unintuitive response remains a long-lasting mystery. Here, we uncover its mechanism. Cross-database analysis identifies borohydrides as a particularly state-diverse and composition-responsive material family. Structure prediction and ab initio molecular simulations reveal that increasing NH₃ loading increases the direct Li–N coordination number while progressively decreasing BH₄⁻-associated contacts in the local Li environment. Near x ≈ 2, these contributions are most balanced among the simulated compositions, and the sampled Li–N/N⋯B coordination landscape is broadest. Further ammoniation produces Li–N-dominant coordination and slower BH₄⁻/NH₃ contact renewal, accompanying recovery of a more rigid ammoniate state. Pressure–composition isotherm, ¹H and ¹¹B nuclear magnetic resonance, and Raman measurements support this non-monotonic state evolution and associated BH₄⁻/NH₃ reorganization. These findings transform ammonia-induced liquefaction from an empirical phase anomaly into a competition between native-network disruption, mixed-coordination frustration, and ligand-built network reconstruction, providing a framework for chemically switching between transport-favouring fluidity and stability-favouring rigidity in hydrogen-rich materials.