Ethan Hawke (Jesse): I don't think anybody does [change]... I mean people don't want to admit it, but its like, we have these innate set points... you know like nothing much that happens to us changes our disposition.
Julie Delpy (Celine): Really? You believe that...
Jesse: I read this study where they followed people who'd won the lottery, and people who had become paraplegic... so, you'd think that one extreme is going to make you euphoric, and the other suicidal... but the study shows that after about six months... right as soon as people got used to their new situation, they were more or less the same.
Celine: The same !?
Jesse: Well... yeah. Like if they were an optimistic jovial person, they're now an optimistic jovial person in a wheelchair. If they are petty miserable assholes, they're now a petty miserable asshole with a new cadilac, a house and a boat.
Celine: So, now I'll be forever depressed no matter what great things happen in my life?
Jesse: Definitely (with a smirk)
I guess if I put on my preachy hat, I'd have to say that general happiness, and contentment is something that can only be achieved through within oneself. No amount of materials (consumerism, counseling and anti-depressants will qualify) can bring about real happiness in a human being, for the fact that they are finite and will eventually end (and will follow the pattern set in the dialogue above).
From what I understand of it, the single most important ingredient to happiness is acceptance. Acceptance is an umbrella for a variety of sentiments, which would include 'I don't give a shit', 'Its okay.', 'Who cares!', 'Its all part of life...', and many more.
As a complement to acceptance comes in handy the existential idea of choice and consequence. Wherever one has a choice, it can be exercised to change the circumstances. An when one does not have a choice, one can accept the circumstances in order to eliminate any possible conflicts arising from them.
Again, all this pseudo philosophical mumbo jumbo is a lot harder in practice than in real life as I have found. But, to speak from experience, it does enhance the quality of being, and hence, the quality of life.