Serenity is not just pleasant. It is medicine. Krishna says: in that state of prasada (inner clarity), all sorrows are destroyed. And the serene mind's intellect rapidly becomes steady. One thing feeds the other — clarity removes sorrow, and the absence of sorrow steadies the intellect further.
Think of it as a virtuous cycle. Sense-discipline leads to inner calm. Inner calm dissolves sorrow. The absence of sorrow sharpens the intellect. A sharp intellect supports even deeper calm. The wheel turns upward, just as the chain in 2.62-63 turned downward.