The best efforts of self-defense can be made ineffective if we fail to detect the danger. It doesn't mean they are stupid. Some people find a joke in something you said that you may have missed.
Danger works the same way.
Some people will detect a danger faster than others. Ask any police officer about that. They will tell you they find some officers are incredible at knowing when something is wrong and others seem to never have a clue as to a danger when it may be obvious to most others. If you are in the average category which most of us are, then you will have a reasonable chance to protect yourself from harm. It is vital you take a long hard look at your perception of dangers.