He does it so he can explain his life.
Jay gastby rents a house next to him asks his chauffer to invite nick at one of gastby's parties
"Why" is the question to ask to help identify the reason for an event that happens in a historical text.
If you ask "why" an event happened, you will get to better understand the reasoning around it, what prompted the event, and some of the personal/political emotions or decisions that surrounded the event.