Fun is not the right word. Rewarding is the right word.
That rush of good feelings from giving the right answers and conquering mountains of information. Those are the highlights.
That is my answer so far, at least.
It's like you're an Olympic athlete marching toward a goal. Say you're a gymnast practicing and hurting your body 6+ hrs a day.
Ask the gymnast: "Is it fun?" How would s/he answer? That's the wrong Q to ask, almost. "Is it rewarding to learn a new, more complicated move?" "Is it rewarding to feel yourself improving every day and doing things you never thought you could do?" "Is it rewarding to compete, do well, and realize that one day you will get a shot at the ultimate competition?"
Yes, yes, and yes.
In our case, our goal is successfully completing med school and landing good residencies.
There's plenty of reward at the end and dotted all along the way. There's plenty bits of happiness to be found too. Fun? Arguably... yes. Not quite the right word.
I think it depends on what type of person you are. For MOST people, studying that much and working that much of a grind is most definitely NOT fun.
But keep in mind... you are NOT like most people! Most people don't get in med school. You are in med school.
Most people are not Olympic athletes who put themselves (willingly and eagerly) through 6+ hrs of training a day.
The people who do clearly have a love for the endeavor and a strong, driven spirit.
For these people, there's plenty of reward, happiness, and yes, moments of fun, to be had.