This is such a deceptive work of art. The first episode led you to believe this is a drama about office politics and some kid getting the short end of the stick, and it was uncomfortable to watch IU getting beat up.
Other reviews have already mentioned how wonderful it was to see their relationship grew and the unshakeable bond they have towards the other, that touched the very core of our beings.
I want to point out the many deep threads that upon rewatching it may be worth more thoughts, such as the juxtaposition of the monk's wisdom and his own personal struggles, ML's pristine good personality ending up hurting himself more and was not worth as much as he thought, and how he and IU grew together through all the struggles.
I personally thought some of the threads didn't work as well such as the struggling actress who seemed out of place in terms of reality of the drama, but they don't detract from the soul of the drama.
They even spend time developing powerful characterization for secondary characters. The grandma, she conveyed so much and everytime she's on screen it's a gem. The bully started out as a cardboard villain, kicking IU was unbearable to watch. But the back story they crafted for him revealed deep conflicts within himself, even though it was in service of a plot twist, it was still believable.
This is a very powerful human drama and one of the most meaningful experiences I've had in watching any drama.