I think this is really the core of it:>>3391889
You see a guy who's in deep shit but still doing whatever he can to make his miserable situation somewhat better, despite his numerous shortcomings and unluckiness. The cherry on top that he's dissociating so hard, he can't even tell what's real anymore.
He finds out he's adopted, his parents abused him, his idol/father figure ridicules him on live TV and the other one punches him in the face, he gets jumped by punks, gets fired from his job, betrayed by his friends... it's honestly almost funny just how much suffering Arthur has to endure one after the other like a conga line.
But still the real kicker, is that he honestly tried. He took his mindbending medicines, he went to his therapist until they cut funding, he took the beating in pride (and made a joke with the water flower), he still didn't gave up his dream after getting fired, he tried his best at the standup, despite bearing the pain of his condition.
It literally makes everyone want to give a hug to the poor clown.