Confidence is good. But overconfidence can lead to a disconnect when it comes to real life violence. It is too easy to fall into the trap of thinking aptitude in the dojo means one is automatically safe in the real world.
I've seen this in adults, from time to time, but where I've really noticed it lately is with teenagers who have put in a couple of years, and achieved some level of success when it comes to class. You see the confidence increase, which is good, but sometimes there develops a slight arrogance, and a belief that a high belt colour means you can hold your own with anyone, in any type, of situation. But it's just not true. It's not the way life works.
Unfortunately, sometimes, depending on the student (and to a large degree the teacher), success in the dojo can be comparable to being book smart at school. Good grades don't necessarily mean intelligence outside of the classroom.
Once, for example, I was watching a class where the teenagers were having semi-formal grappling/wrestling matches. A new kid was there, and he, because of his size, was paired up with an experienced brown belt in the class. Everyone was thinking, he's going to get destroyed by the senior student, and the group gathered around in a circle to watch. However, within seconds of the beginning of the match, the new kid pinned the brown belt on the mat and the match was over. And then he did it again.
Luckily, it was just a friendly match in the dojo.
In fairness, most of us who have put in any amount of time training have likely felt increased confidence because of our increased abilities. I get it. But what some students don't yet realize, and it may take many more years of training and real life experience to understand, is that humility needs to be one of the key attributes discovered along the road of training. Use your confidence, but don't waver from caution; don't think yourself invincible, because no one is. After all, if we are to never underestimate an opponent, it means to never overestimate yourself.
Real life violence cannot be duplicated in a dojo for the sake of training. Real violence is ugly and unpredictable. It should be avoided, if possible.
I guess I just worry some of these students, with their new-found confidence, will put themselves in bad situations because of, well, being a bit naive. Because confidence can only take one so far, and some of the book smarts have to be interpreted and/or translated into realistic thinking.