it's that 'I am gonna hurt you bad' attitude..... but it's not natural born quality for every player.
how do you develop this? especially for the juniors (but not limited to)
By attitude, if I understand, you mean an overall approach to game play and strategy, isn't it?
As I stated times a many, the question of aggressiveness is not whether it can be useful or if it should be promoted as a tool in the pursuit of success... obviously, it can work. What is actually problematic is finding the exactly right measure. Given your question, I assume that you do present excessive aggressiveness on the court. But erring on either side is equally sinful, as the player's bible goes.
So, how do you get from doing too less to doing enough, bearing in mind that it would be counterproductive to do too much?
To be consequent to the context in your choices, you have to know yourself properly. It would enable yourself to understand on which side of the perfect measure you tend to err and how your reactions vary from time to time. Experience, if sufficiently diverse and properly revised, can bring you to be a better judge of your own thoughts. That's essentially how we choose what we study, what sort of job we would enjoy doing or what kind of person we aim to become... all of these choices and the engagement we take toward them mark our entry in the adult life and it is equally true for tennis: your identity as a player is also a construction and it is built through your game time and it is modified through the entire array of social relationships you take part in as you live.
The reasons which may explain your excessive inhibition (if it truly is the case) are probably emotional. I would need more information and time to figure out where's the issue -- it can be big or small, but there's an issue which underline your tendencies. It's not simple and you won't cure it with magical solutions some guru will bring on the net... to cure a disease, you need to attack the causes, not the symptoms, but gurus only talk about symptoms.
One sure way to partly compensate for this tendency would be to tune up your stroke to make sufficiently reliable and to practice enough to conceive yourself of its efficiency.
One thing that you should keep in mind during matches is that you make choices based on specific factors and you try to make them so that they work most of the time. If it fails in a peculiar case, it's not a good reason to throw the whole play book by the window. Stay clam, evaluate the decision and its coherence with the context and, if it seemed logical, stick with it. Players often start to fool around with everything during a match and some of their mistakes revolve around this total lack of coherence in their plan... specifically because they have no plan.