Great Answers! Like a lot of questions I think you can interpret it a lot of ways but after decades of watching Chinatown I'd like to inject this interpretation...
Opening scenes of Jake's new career paint the picture of him as a pretty disreputable opportunist. Photographing unfaithful partners and facilitating their Hollywood divorces are stereotypical examples of the work of mercenary private eyes.
Jake is a former police officer with no love lost between him and a few members of the LAPD. The noire sensibility of the film and and the real history of the LAPD indicate that Jake was either an incredibly corrupt cop for not getting along with them, or an honest man that chose what some would view as a sketchy way to make a living rather than dealing with the hypocrisy of his former career. The latter is what his interaction with his former colleagues seems to point to.
The catalyst for the change in Jake, the tipping point, is painted ominously as events that occurred in Chinatown. While the events are never delineated, Jake is clearly concerned about operating in that area, and people that know him know that he should be.
The Chinese characters in the film are all (with the exception of the gardener) party to the main coverup of the film. The virtual imprisonment, including the drug induced somnolence, of Katherine occurs with either their tacit or active participation. When Jake decides to help Evelyn and Katherine escape, it is the Chinese butler and other household staff that get enlisted in the effort to hide her from Cross, the father of both women.
Jake makes the choice to send them to Chinatown. He reintroduces the moral conflict that drove him away from the police force and into his sketchy new life. Rather than walk away from the old conflicts and uncertainties in the way that he did by hiding from them as a disinterested private eye, he confronts the old demons head on.
The dark climax of the film reinforces the central message that fate is inescapable. No matter what our motives or our actions the cycle of life confronts us with the consequences of our destructive nature. "Forget it Jake- it's Chinatown" means there is no escaping those consequences. Evelyn meets her doom and Jake is forced to bear witness to it instead of hide from it the way he tried to by leaving the LAPD.
Apologies for the long winded answer, just in case you didn't know...LOVE THIS MOVIE.