One of the things I teach is Personal Adjustment, which is a poor name for what the class really is: a class in positive psychology. Yesterday, I covered traits and happiness. The whole thing about that is that we have personal genetic traits which might influence our happiness.
One way to measuring enduring traits is the Big Five Personality Test, which has been tested for correlation not only to the traits measured but to happiness. The Big Five mentioned in the title measures five dimensions (see picture below):

Some of these dimensions are linked to happiness as follows (Judge et al., 2007):
- Higher openness to experience correlates to higher happiness
- Higher extraversion correlates to higher happiness
- Higher emotional stability correlates to higher happiness
- Agreeableness and conscientiousness do not correlate to higher happiness
In other words, if you score low extraversion (i.e. score as an introvert), you will experience less happiness than someone who scores high in extraversion — and half that gap is unchangeable; it is a trait you have that won’t go away.
The good news, though, is the other half (on average) of that gap can be changeable. By pushing your comfort zones, you can recoup some of your happiness in openness and extraversion. By learning to manage emotions, one can increase emotional security.
So some people are happier than others, but it’s possible to approach a higher level of happiness through self-work, and that’s a good thing.