I've been working with athletes for years. The most common struggle I encounter is a lack of aggression. It's prevalent. So much skill and excellence being wasted due to passivity. Lack of aggression always comes down to these three things...
1) Fear of Humiliation
There is a risk that comes with aggression. When you assert yourself, you are vulnerable. People are not aggressive because they fear failing in front of an audience. It is much safer to stay in the shadows.
They don't fear missing shots, they fear people seeing them miss shots. It's the potential embarrassment that shackles their aggression. They don't take it strong to the hoop because if someone blocks them it will be humiliating.
2) Fear of Rejection
Another one of the top reasons players are not aggressive is their fear of being socially rejected. If they relentlessly attack and mercilessly dominate their opponent, they will be disliked.
The number one human need is acceptance. Nobody wants to be seen as the jerk. Being aggressive is a fast track way for people to not like you. The most aggressive people seem to relish being hated. They like it. But that is rare. To most, the potential of being disliked is not worth the risk of being aggressive.
3) A Lack of Self-Worth
This is the most common reason people do not assert themselves. Deep down inside, they believe they are second class. They don't feel that they have the right to shoot the ball or drive it.
If you don't believe you deserve to be the victor, you will find a way to shrink back. You will always live to the label that you give yourself. If you have an internal label that says you're less valuable than others, you won't assert yourself. I see it everyday.