music, comfort
樂 centers on harmony and ease: music as structured sound that brings comfort, and the resulting feeling of relaxed enjoyment.
樂 is a pictograph of a stringed instrument with a wooden body and silk strings, later extended to the comfort music brings. The modern form preserves the wooden base 木 and the stringed top elements.
Picture a wooden instrument 木 with strings and a drum-like top: the whole shape 樂 represents music, which brings comfort and ease.
For ガク, imagine a gaku (a traditional Japanese musical instrument) playing soothing music: gaku -> ガク, and the sound fills the room with comfort.