We can all think back to a time when we were substantially different people, value-wise, from the people we are now.
Agency, as distinguished from mere behavior, is marked by practical rationality. Insofar as becoming is something someone does, and not merely something that happens to her, she must have access to reasons for becoming the kind of person she will be.
The name I give to the rational process by which we work to care about (or love, or value, or desire...) something new is "aspiration". Aspiration, as I understand it, is the distinctive form of agency directed at the acquistion of values.
Proleptic reasons are...the reasons that rationalize large-scale transformative pursuits. A proleptic reason is an acknowledgedly immature variant of a standard reason.