Why GetComponent()?

Why would you want to pass an object into GetComponent? That would mean every time you want to use GetComponent you need to make a new object.
You can use the static keyword, what are you talking about???

I have a class that raycasts from the camera to get the position in world space that the mouse is over. In order to get the result for my other classes I need to make a reference. When I call

 $$anonymous$$ousePosition mousePosition = new $$anonymous$$ousePosition();

I get an error telling me you can't use the new key word with object inherited from $$anonymous$$onoBehaviour. And if I make the $$anonymous$$ousePosition() class static I get a warning saying I shouldn't, and the script fails to work at runtime. The only thing that works is:

 $$anonymous$$ousePosition mousePosition = playerCamera.GetComponent<$$anonymous$$ousePosition>();

I guess what I am asking is why I can't use the new keyword and why it makes errors when you use static with monoBehaviour.

Unity use an object/component model. Game object get components. Components are instances of a class attached to game objects. GetComponent() does not create a new component, it get a reference to an existing component. When you drag a script onto an object, or use Add Component button, or when you use the Component menu, you are creating and attaching an instance of a specific class to a game object. If you wanted to dynamically create a component at runtime, you would use GameObject.AddComponent ins$$anonymous$$d of 'new'. But note you would be creating that object as a component of some game object. You also use Awake() or possibly Start() in place of a tradition class constructor in Unity.

Note you are free to use new and constructors for classes you don't derive from $$anonymous$$onobehaviour.