Why should you never underestimate your opponent?

Answer 1

Here's the thing; You can never be sure of the things you don't know. Your opponent may have something up their sleeve that you didn't see coming, no matter how weak they appear. By assuming your opponent could beat you, you are better able to objectively think about all of the many ways in which they could potentially trump your argument or battle strategy, and prepare for those situations, no matter how unlikely they seem.

Much of the information (or training, if you are in sports/battle) you must gather before you face your opponent will never be used, but knowing what to do in the event that you are challenged where you are weakest gives you the best shot at winning in a worst-case scenareo. Never assume you have won until it's over, and you will have the best shot of doing so. If you underestimate your opponent, you give them the opportunity to see something you do not, and attack you in a way you are not prepared to defend yourself against.