I have always had health insurance.
However, I grew up poor and had many friends who did not have health insurance for various lengths of time, almost always due to poverty, unemployment, and sometimes a thought that healthy young people don't need it. Real things that happened to three individuals among my uninsured friends:
Non-Hodgkins lymphoma in his 20's
Ruptured spleen (weird accident involving a ski pole to the mid-section . . . don't kid yourself: he is a better skier than you are)
Blood clot in leg that led to hospitalization and then a rehab center stay (an older friend in his 50's)
The first one was the scariest . . . friend right out of college was unemployed, couldn't find a job and so decided to run for congress, and while he was running found out he needed chemotherapy. Pretty awful situation. Spleen guy was saved by ski patrol realizing it was truly an emergency situation. Last guy ended up homeless because he couldn't pay his rent while in rehab, but once he was released found a friend to let him room for free in exchange for letting the landlords dogs out and feeding them during the day.
All of these people thought they were perfectly healthy and didn't need insurance. Only the ruptured spleen guy ever did anything especially risky in terms of becoming injured. Passing on health insurance is a bad call. Basically if you have enough for minimal food and have even a leaky a roof over your head, then health insurance is next in the USA.