I have a two-year-old son, and I can tell you that patience is a learned behavior. My boy currently takes what he wants and reacts to virtually all of life's circumstances with a rash, emotional response. More often than not, this is a positive thing, as he finds joy and discovery in life's simplest offerings. Sometimes, however, his rashness works to his own detriment, and he ends up getting into trouble that he might easily have avoided had he only stopped to check himself first. His mother and I are charged with the task of helping him form the ability to think before he acts, to discern logical and beneficial action from rash behavior that might lead to undesired consequences.
Rash reaction is a hard habit to break, though. I see it working against my students' self-interests weekly. When kids are confronted by security staff about, say, a dress code violation, their initial reactions are invariably belligerent. I understand not wanting to be told what to do - especially as it pertains to freedom of personal expression, but what the students don't understand is that their best option in such a situation is to respectfully address the security staff and simply correct the "problem" as directed. When you mouth off to security, if you give too much "attitude", well, now you have a dress code violation and an insubordination issue as well.
So, how does this help us understand the quote? Author, Willa Cather gives us "boys" who have not yet learned patience. Unable to "choose their battles wisely", these boys end up fighting a lot of battles they could have just as easily avoided (and should avoid) if only they knew how. Hence, when crossed, they don't think - they react... often to their own detriment.