As you can see from two arguments, reason is a key word. Reason has two meaning;
Reason
1. A motive, ground, or cause for something. Example; He had a good reason to quit his job.
2. Good sense or judgment. Example; She made a very reasonable decision.
An argument, then, offers reasons for accepting its claim.
In a good argument, those reasons are reasonable.
They’re based on evidence or good sense or judgment