There are four stages to a person’s career:
In Stage 1 you are enthusiastic about your work, but inexperienced (start of your career).
In Stage 2 you are both enthusiastic about your work and have gained experience (top of your career).
In Stage 3 you’re tired of your work, but you are also still competent/experienced (maintenance stage).
In Stage 4 you are sick of your work, and because you haven’t been motivated to keep up with your profession, you are now, once again, inexperienced relative to the state-of-the-art in your field (end of career).
Some people manage to avoid the latter stages of this progression by purposefully staying engaged in and enthralled by the work they do. The point of this aphorism is simply to challenge a person to ask themselves whether the sentiment expressed may be applicable to their lives, and if so whether a change of course might be needed to prevent the all-too-familiar career outcome described.