The why's often involve the human element. When a person chooses to do something (or to not do something), there has to be a reason for it. Some thought must have occurred and determined that it was all right.
The desire to know 'why' is the desire to understand how the human mind works. But sometimes we find out that the answer is unsatisfying. Sometimes a person's answer to 'why' is, "Because I can."
Yeah. Not every action has a grand plan. Some of them are completely whimsical. They are done just to do them, just to see the result, just to kill boredom. They are the most frustrating people to encounter, as they are truly unknowable. They also can be terribly frustrating characters to write for, and for the same reason.