I need some help with camera rotations

Well, I am trying to make an effect on my 3d top-down shooter game. This effect includes the camera's rotation. Basically, what I want to do, is to keep the camera at the player, but when I move the mouse on the screen (lets just say I move it top right), the camera should extend towards that point, but never actually look away from the player.

This works, as long as I only use either the X or Y axis. If I use them both, the entire camera freaks out and starts swinging wide everywhere.

My code this far (It's on the camera):

 using UnityEngine;
 using System.Collections;
 
 public class PlayerCamera : MonoBehaviour {
 
     public Vector3 offset = new Vector3(0,0,0);
     public float cameraReach = 16;
 
     float xRotation = 0.0f;
     float yRotation = 0.0f;
 
     GameObject player;
 
     void Start () 
     {
         player = GameObject.Find("Plr_Player");
     }
 
     void Update () 
     {
         //xRotation = ((Input.mousePosition.x - (Screen.height / 2)) / cameraReach);
         //yRotation = ((Input.mousePosition.y - (Screen.height / 2)) / cameraReach);
 
         xRotation = (-transform.rotation.eulerAngles.x + (((Input.mousePosition.x - (Screen.height / 2)) / cameraReach)));
         yRotation = (-transform.rotation.eulerAngles.y  - (((Input.mousePosition.y - (Screen.height / 2)) / cameraReach)));
     }
 
     void LateUpdate () 
     {
         transform.position = player.transform.position + offset;
         transform.rotation = Quaternion.Euler(yRotation, xRotation , transform.rotation.eulerAngles.z);
     }
 }