Some men are bald through choice. I had a work colleague who was bald. He often made self-deprecating jokes about his own baldness, so I followed his lead and often teased him about it. Then one day he told me the real reason he was bald. I felt terrible for having teased him. He said that his fiancé had got cancer and lost her hair to chemo. To make her feel better, he shaved off all his hair while she was in the hospital. Then she died.
He has kept his head bald in her memory. Even though he met a new fiancé whom he later married, he has never grown back his hair, and just keeps shaving it away. I do not know if this is a common thing (men shaving off their hair to show solidarity for cancer) but for all I know, there may be other bald men out there who shave their heads voluntarily for personal reasons.
I have heard of women shaving their heads in solidarity with a female friend who's lost their hair because of cancer treatment. I also know of men with long hair who've been sponsored to have their heads shaved to raise money for cancer charities.