Aggressiveness is something I teach my athletes from the beginning.
In almost every sport, the aggressive athlete has the upper hand. Because of this, we teach "be aggressive" at the youngest of ages. When the skills haven't been developed yet, aggression usually comes out on top.
When my daughter started playing soccer at five years old, this was my only coaching point: be aggressive. It was in the hope of instilling in her the courage to go get the ball!
There are plenty of athletes out there who are highly skilled and great practice players, but when the lights come on, are submissive. Fear of failure seems to be the primary culprit. It forfeits their edge, regardless of their skill or talent.
Many times you'll see the aggressive player win out over the talented player because of their willingness to compete and be proactive.
Aggressive, by no means, is uncontrolled behavior. Instead, it is controlled and relentless. This is the guy coming downhill at you all game. They exhaust their opponents because they never let up. There the person that when they check into the game, you tie your shoes a little tighter.
I believe there are a few ways to help push that non-aggressive kid (who has skill) to be more aggressive.
1 - Help them learn to accept that failure as a way of getting better. They don't have to be perfect. They can mess up. In fact, encourage them to mess up but do it aggressively.
2- Award them for being aggressive. Make it a priority. There are many ways to reward an athlete. The best usually comes through encouragement and telling them how hard and aggressive they played.
3- Get them stronger, faster, and more powerful. I may be biased here, but I've seen more tentative kids turn into monsters on the field or court after enlisting into a good strength training program. Strength helps give them that edge and helps them believe; they now are the aggressor.
4- Help show them aggressiveness is fun and is the only way to play. Aggressiveness means you're competing. If you're not competing, then what are you doing? It's no fun to be the best practice player. It's fun to be the most feared in-game player. Teach them in any way possible about this. Talk, read, and watch videos about great players in every sport, how they attacked the game, and what their mindset was.
5- Show them love. No matter what, kids need to know they have support. I tell kids all the time, "your performance has nothing to do with how I like you as a person." Kids who have positive support and can separate themselves as a person and an athlete tend to play with less fear. They don't feel their whole life is on the line every time they compete. They can relax, go hard, display their skill, and win or lose; they know they're still loved.