Obviously Jesse was disturbed when Todd killed that kid on the bike in Breaking Bad season 5. Then he decided to stop cooking and asked Walt for his share of the millions. He wanted the money so that he can live the way he wanted. But the moment he gets the money, he shuts the door, sits down and start crying. After that he goes out and starts throwing the money. I did not get this. What exactly happened when Walt gave him his money?
Jesse didn't completely change. Jesse was very sensitive from the begining. The shooting of boy, by Todd, definitely had an impact on Jesse. He not only asks Walt for money, but also asks Saul to help him give away his money. When Saul refuses, he starts throwing the money and gets arrested.
Refering to the OP's comment what exactly was serious about this instance?
Earlier incidents also traumatized Jesse to a lesser degree, like the killing of Andrea's brother by drug dealers. But at this point, when Todd killed the boy, maybe he had had enough. Jesse faced several such traumas during the series following which he always recovered, but we can see that he reached the breaking point after this incident. Before this, everything seemed to be fine. Gus was gone, they were doing independent business, Jesse was living with Andrea and Broch, but then this incident occurs and Jesse just seemed to have enough of it. There were a series of incidents, but this one acted as the breaking point.