It's all [okay, mostly] in your head.

Much more at the link.

Live Life Aggressively!: Is Happiness Determined By Our Genetics?:

"...it's difficult to achieve happiness via changing external conditions. Our brains are good at adapting to situations, good or bad. This isn't so hard to understand, think of any important goal you've achieved--remember how anti-climactic it felt? This is the problem with being overly attached to end results: we place too much pressure on achievement changing our mindset...

Of course, no achievement will ever be enough., which is why people unconsciously stay in the anticipation phase and avoid achieving their goals...

Does this mean we shouldn't bother with goals? Of course not! Goal-less-ness is the path of the cop out. People who claim that everything is illusion are unmotivated people looking to avoid growth and change. The key is setting goals and achieving them for the sake of doing it... According to John Elliot, author of The New Science Of Working Less To Accomplish More, we perform better when we're fully in the moment and unattached to outcome: life's fullest moments can be reduced to those moments in which we're fully present with no thoughts of past or future. These are the moments in which we're fully alive and time seems to stand still...

Everyone wants to be happy whether they realize it or not and even if they don't want to admit it. We need to realize that happiness isn't a result of focusing on conditions."