My script does not work each time i enter Unity unless i add it to the object again. any solutions?

I there i am pretty new at unity and found this script online and made a few edits(Javascript, and i do not understand the math behind it): #pragma strict

 function Update () {
     var pos = Camera.main.WorldToScreenPoint(transform.position); //A variable for the position of the camera on the world
     var dir = Input.mousePosition - pos; //the mouse position minus the camera ray
     var angle = Mathf.Atan2(dir.y, dir.x) * Mathf.Rad2Deg; //a equation that will find the correct angle to set the rotation from object to mouse
     transform.rotation = Quaternion.AngleAxis(angle, Vector3.down); //the change of the rotations Y axis
 }

For more info on what i am doing:

• I am redoing the unity space tutorial game but added this to my ship so it can turn and shoot the bolts out

• this code works for the player every time i enter unity but if you saw the tutorial you would know there is the "Shot Spawn" under it.

• Shot spawn turns with the player ship whenever i delete the script as a component and bring it in

Does anyone know what is wrong with it so whenever i open Unity it can just stay and work?!?!

EDIT: I found a solution where i just make the code look for the player and not the shot spawn, and i made the bullets come out of the player to make it work every time