Justin Wehr summarizes the research on "interest." According to one paper by Paul Silva he dug up, something is interesting if it is: a) new, complex, or unexpected, and b) comprehensible.
Silva's extrapolation for writers:
According to educational research, the largest predictors of a text’s interestingness are (a) a cluster of novelty–complexity variables (the material’s novelty, vividness, complexity, and surprisingness) and (b) a cluster of comprehension variables (coherence, concreteness, and ease of processing). Intuition tells us that we can make writing interesting by "spicing it up"; research reminds us that clarity, structure, and coherence enhance a reader’s interest, too.
Interest motivates learning about something new and complex; once people understand the thing, it is not interesting anymore. The new knowledge, in turn, enables more things to be interesting. … In a sense, interest is self-propelling: It motivates people to learn, thereby giving them the knowledge needed to be interested.
This would suggest that sometimes you're not going to be interested in something right out of the gate — you first need to acquire some knowledge in the area, some experiences, some expertise. Map this to careers and you arrive at Cal Newport's view that you should try to generate passion at work, not find your passion.
You can be interested in things but not be happy:
Second, interest and happiness connect to different abstract dimensions of personality. Interest connects to openness to experience, a broad trait associated with curiosity, unconventionality, and creativity. Happiness, in contrast, connects to extraversion, a broad trait associated with positive emotions and gregariousness.
When I think about the people I most enjoy spending time with, they are high on two scales: interestingness and humor.
Here are Andy McKenzie's thoughts on the link between interest and the potential for a reward. Here's Andy on why happiness and sadness on are different dimensions.