The goal-gradient theory explains that when people get closer to a goal, they increase their effort to achieve the goal faster. In other words, people are motivated by the idea that they are close to completing their aspiration.
Relevant Academic Papers
Kivetz, R., Urminsky, O., & Zheng, Y. (2006). The goal-gradient hypothesis resurrected: Purchase acceleration, illusionary goal progress, and customer retention. Journal of marketing research, 43(1), 39-58.