continuously Find closest with tag, in update.

How can I continue to update the closest object with tag? I want to know what object is closest at all times, but it never changes, just sets the closest once and it stays at the same spot.

I appreciate any help here. Thanks

using UnityEngine; using System.Collections; using UnityEngine.Networking;

public class ESportBallMovement : MonoBehaviour {

 GameObject[] ballWayPoints;

 GameObject closest;
 private Vector3 targetPos;
 float distance = Mathf.Infinity;
 public float speed;
 float step;

 void Start()
 {
     ballWayPoints = GameObject.FindGameObjectsWithTag("ballWaypoint");
     step = speed * Time.deltaTime;
 }

 void Update () 
 {
     FindClosestWaypoint ();

     //sets the target position and moves toward it
     targetPos = (closest.transform.position);    
     transform.position = Vector3.MoveTowards (transform.position, targetPos, step);
 }

 GameObject FindClosestWaypoint() 
 {
     //populates the array of ballWaypoints
     foreach (GameObject ballWayPoint in ballWayPoints) 
     {
         //finds the distance between ball and the ballWaypoints
         Vector3 diff = ballWayPoint.transform.position - transform.position;
         float curDistance = diff.sqrMagnitude;

         //finds the closest ballWaypoint
         if (curDistance < distance) 
         {
             closest = ballWayPoint;
             distance = curDistance;
         }
     }
     Debug.Log (" Current Waypoint:" + closest.name);
     return closest;
 }

}