The Third Elder really loves drinking.
Initially, she thought drinking like others would seem quite dashing, but as she drank more and more, it slowly became a habit, leading her to fall in love with that carefree, forget-the-world feeling, drinking whenever she felt like it.
The reason she liked this feeling was to dispel the boredom of life.
Living atop the mountain, knowing it's hopeless to break through to Celestial Awakening, continuing cultivation seemed meaningless; after centuries of cultivating, even the famous mountains and great rivers no longer felt novel, and planting flowers and trees were no longer challenging, making her days feel utterly dull, as if she could see the end of everything at a glance.
Since that was the case, she might as well drink herself to oblivion, and exclaim upon waking, 'Ah, that was nice, I slept for three days.'
