Let us disassemble樂 to see how it is composed. You might have found top partfamiliar as we just learnt it in our last lesson of "丝", top middle part is 白 (thumb) and lower part is 木 (wood). Typical Chinese musical instruments are made of strings with silk and wooden frames, as show below: more can be found at https://en.wikipedia.org/wiki/List_of_Chinese_musical_instruments
|Chinese Musical Instrument - 古筝|
|Playing Musical Instrument|
As hearing wonderful musics is an delightful entertainment, 乐 | 樂 was extended to mean Happy, Glad, Amused, Delightful as adj, and Happily, Gladly as adv.
Let us examine evolution history of 乐 | 樂 to confirm our explanation above: (image get from http://www.vividict.com )
|Learn one Chinese Character a day – 乐 | 樂|
樂 is then another proof that 白 refers to thumb as its original meaning.
For the Simplified version "乐", you might agree that it does look like a piano opened for playing.
Now it is time to enjoy a Chinese calligraphy with "乐 | 樂" inside: