That's because we are constantly learning and exposing ourselves to new information and ideas that can change our ways of thinking. We are not static creatures--our very sense of who we are shifts and mutates with the passage of time and accumulation of experience.
We need to recognize those changes as we try to live the best possible lives. Recognizing that the motivations that were once so powerful may not be almost inconsequential is essential in trying to reach our potential.
You should revisit your personal journey to self-fulfillment regularly. Take time to consider whether the motivations that set you on your current path are still as strong as they once were. If it appears their power has decreased, do some soul-searching about what currently "lights your fire" and why. You can then use the discovery of new and changing motivations to your advantage by making a conscious decision to allow yourself room for positive change and to adjust your individual course accordingly.
Many people find themselves "dead-ending" as they work toward self-improvement. One of the reasons this happens is because they develop a plan and stick to it with tremendous determination. That dedication is enviable, in many ways, but it becomes counterproductive it leads one to continue acting consistently with their "old" mindset even after their thinking (and beliefs) may have changed.
Check in on your motivations regularly. If they haven't changed, that introspection will help reaffirm them. If they have, the investigation will allow you to adjust in a way that matches your increased experience and wisdom.