Uncle John is Pa Joad's lonely older brother. He's a bit of a recluse, but he's got a big heart nonetheless. John used to be married once upon a time, but one day his wife complained of a pain in her gut. John told her she simply had a stomachache. A day later, she died of appendicitis. Every since then, he's felt incredibly guilty and lonely. Now, anytime anyone has an itch or a cut or a scrape on their elbow, John runs to get the doctor.
John also likes to do nice things for people. Sometimes he'll slip candy into his nieces' and nephews' pockets or leave candy beside their beds. He lives a pure life: "He ate little, drank nothing, and was celibate" (10.47). However, there are times when his buried emotion rears its head:
[…] but underneath, his appetites swelled into pressures until they broke through. Then he would eat of some craved food until he was sick; or he would drink jake or whisky until he was a shaken paralytic with red wet eyes; or he would raven with lust for some whore in Sallisaw. (10.47)
He's a tortured soul, that's for sure. We wish that someone would give him a lesson on the importance of moderation in all things... but it's pretty clear that the ship of John's health and happiness has sailed. Poor dude.