Jesus is their friend, but He now wants to become their lover. He commands them to choose to be zealous, or hot (like the healing waters of Hierapolis), and to have their life defined by repentance—but with this reference to being zealous and its overtones to the healing waters of Hierapolis, He is hinting at His purpose for standing at the door and knocking. He desires their deep inner healing.
Yahweh has taken His position at the door, and He has always been there. Yahweh Shammah is there and has never taken His eyes or His attention off them or us. He has been waiting for the right time to knock. But He now knocks. Now is the time.
Starting in life, the doors to our soul are open, but as we experience pain and trauma (wounds) from our family, friends, loved ones, and from the world at large, we start to shut those doors determined to protect ourselves to ensure that we will never be hurt that way again by those who should have protected, cared for, and loved us.
Jesus loves us too much to leave us in that condition. Therefore, He hangs out in the corridor of our soul watching and waiting, and when the time is right to deal with the troubles in a specific room, He goes to that door, and He knocks. His goal is to secure our willingness to open the door so He can establish relationship, fellowship, and healing in every room that we have shut tight.