The question regarding why an individual chooses one course of action and not another can only be answered in one of two ways: Either (a.)there is no reason why the course of action is chosen, or (b.)there is a reason why the course of action is chosen. If there is no reason why the course of action is chosen, then the choice to take this course of action is completely undetermined. If there is a reason why a course of action is chosen, the choice to take this course of action is determined. If the choice is completely undetermined, then all causal relations are irrelevant. This would include not only agent-extrinsic causal relations (e.g. acting in the world in order to obtain some physical end), but would also include agent-intrinsic relations (e.g. thinking, remembering, analyzing, etc in order to obtain some mental/psychological end). Contrariwise, if the choice is determined, then all causal relationships, extrinsic and intrinsic, are relevant, albeit to varying degrees.
The reason why this is a problem should be evident to anyone who has read the Scriptures. In them, we are told that “faith comes by hearing,” and hearing by “the Word of Christ.” This Word only comes through individuals who have been selected by God to preach, and this is an agent-extrinsic causal relationship. Those who are saved are saved through faith alone in the Gospel preached to them. Intrinsically, the individual must hear and must believe in order to be saved. Extrinsically, the individual must hear the Gospel message delivered by an appointed preacher of Christ. If the an individual’s choice to repent and believe the Gospel are completely undetermined by agent-intrinsic or agent-extrinsic causal relations, then the Scriptures are wrong. However, Scripture is not wrong. Therefore, libertarian freewill is false.
[For the reader who is interested in a more fully developed, and Scripturally explicit, version of the same criticism I offered above, John Hendryx has great article on the matter titled “Eleven (11) Reasons to Reject Libertarian Free Will: A critique of “Why I am not a Calvinist” by Jerry Walls and Joseph Dongell.” You can access the article here.]