Part of fulfillment involves feeling and achieving autonomously set goals. In other words, things which you feel an inherent drive and self-satisfaction with completing. For those with difficult sun placements, this can be troublesome, but as this improves, one's perception of themselves and their abilities becomes more positive, and they will start to feel self-driven.
The Sowilo rune pertains to self-mastery for this reason, because the solar energy flows to all nadis and helps to "centralize" or "unify" the ego in a way, giving a sense of self-esteem, perhaps a self-created "story" of one's self, as well as a better ability to evaluate the opportunities and impact of the self within one's environment. Biologically, this can be seen as an enhanced dopamine response when perceiving the outcome of an activity, like a higher certainty of success and perception of reward.
All of this sets the stage for fulfilling activity and happiness.