Readings & Meanings:
Because I passed my exam, my family were overjoyed.
She, with a gleeful countenance, went to report to the teacher.
口 is the radical, on the bottom, indicating mouth.
The rest is a pictograph showing a serving pot on a stand with a stack of food.
Combine them and you get the idea of a feast, which indicates joy. At least, that's what I feel at a feast.
The sound mimics what the Japanese thought of as the sound of a happy, joyous person, especially in 喜々, KEE-KEE, a high-pitched squeal of joy.
I first learned this word in my winter in Japan. It's part of a folk song that everybody knows:
Yuki ya konko arare ya konko.
Futte mo futte mo mada furi yamanu.
Inu wa yorokobi niwa kake-mawari,
neko wa kotatsu de maruku naru.
You can hear how the song goes here: http://www3.u-toyama.ac.jp/niho/song/yuki/yuki_e.html