I gave this about ten minutes' thought and came up with freedom and -- especially -- possibility. I don't know exactly why those two ideas kept suggesting themselves, but they ring true to me.
I think freedom falls under possibility, and that possibility is the real answer to this question. If you are capable to do the things you want. For example, if you want to travel, it's possible for you if you have the funds to do so, you have the infrastructure to do so (e.g. airports, means to get to the airport), you live in a country where you are able to travel freely (this is where freedom comes in). As you reach higher places in your life, you have a higher ability to do whatever it is you like, and are generally more happy. Someone who has a minimum wage job at a record store without the ability to take vacation time is not going to be happy if they want to travel, but their coworker who desires to drive down the coast on the weekends and make suggestions on music to customers very well may be in the situation most conducive to his happiness. of course, most people's aspirations and desires require more resources to achieve, and that's why money generally does improve happiness to a certain degree, but stops doing so once they've achieved the necessary level for themselves.
Do we really have choice? http://exploringthemind.com/the-mind/brain-scans-can-reveal-... I would argue that every choice you make has already be taken before you think it by your subconsciousness which has been programmed to respond to the environment. Just like you react to heat and cold pain and hunger all choices you make are a response to the environment.
I have a lot of issues with that study and that article, but I can put them aside. I did not argue that we are happy, or that we are free -- that's a separate debate. I do think, however, that freedom (in a grand sense) is the path to happiness (in the abstract).
Happiness is probably a chemical state a balance triggered by things like the perception of freedom and choice. IE having food shelter positive social experiences and psychological well being. I think happiness isn't so much a goal as a side effect or something like the opposite of a stressed state.