Jupiter is conjunct my ascendant and has rulership over 6 planets in my chart as well as my ascendant and part of fortune. "Happy" is not a word I would use to characterize myself. In fact, one of the first threads I created on this forum was entitled, "Why is Sagittarius associated with buoyancy and joviality" because I've never truly related to the merry making traits of Sagittarius and being an exemplar of the sign, it made me call into question the stereotype.
Anyway, what I do relate to is that I am extremely exploratory. I'm willing and able to go to places that people wouldn't dare to (or care to). All for the Sagittarian drive to gain wide ranging experiences and attain "the mysteries" of the world and beyond.
I've been flirting with a thought that this might be the basis of the feelings of "happiness" and it's relation to Sagittarius/Jupiter. Dopamine is a chemical related to feelings of positive emotion.
Astrologically, we see a lot of Jupiter here. Reward and motivation are the most known qualities related, but we also see sexual gratification and arousal (traditionally Jupiter was indicative of a highly sexed nature and I'm sure your familiar with the mythology of Zeus. Even lactation, which brings to mind Cancer and Jupiter's exaltation therein.
This paper I've encountered makes clearer of the connection I'm referring to.
http://ski.clps.brown.edu/papers/KayserExplore_NPP.pdf
We know astrologically that Jupiter/Sagittarius are associated with exploratory, strong belief in the self to tackle the world (the so call optimism), and gambling/taking risks. Well, this study shows the link between these traits and the dopaminergic systems in the brain. Having an internal locus of control sounds an awful lot like the self-belief that supposedly is a characteristic of Jupiterian behaviour, It even makes mention of dopamine and it's role in learning - a very integral part of Jupiter/Sagittarius' focus.
So, while happiness is not an emotion that I'm particularly buddy buddy with in my life, I do display/have a lot of traits that would characterise one with high dopamine levels.