Here's why:
* Schwa sound: Represented by the symbol "ə", it's a short, unstressed vowel sound. It's like a very short "uh" sound.
* "Joy" syllable: In "joyful", the stress is on the second syllable "ful". This means the first syllable "joy" is unstressed.
* Unstressed vowels: Unstressed vowels often become schwa sounds.
Therefore, the "o" in "joy" has the schwa sound (ə).