Press one button, focus all buttons

I am trying to implement an inventory system in unity3d. They are several ways to realize an inventory on gui but i've chosen the easiest.

Every item on gui is represented as a cell(GUI.Button). Every empty cell means empty place in inventory(GUI.Box). When I press one button to chose an item all buttons are getting focused.

My question is why it happens and what to fix to make only one button to be focused? I created a GUISkin based on empty one. Thank you for attention

Here is the code to show an inventory.

void OnGUI(){
    if (visible) {
        GUI.skin = skin;

        GUI.Window(0,new Rect((Screen.width-1024)/2,0,1024,600),InventoryBody,"Inventory");
    }
}

void InventoryBody(int id){
    GUIStyle style = new GUIStyle();

    if (currentItem) {
        GUI.DrawTexture (new Rect(550f,70f,80f,80f), currentItem.texture);
        GUI.color = Color.red;


        GUI.Label (new Rect(700f,50f,400f,300f), currentItem.name);

        GUI.color = Color.black;

        string desc = "Description: "+currentItem.description;

        style.wordWrap=true;
        style.fontSize=18;

        GUI.Label (new Rect(650f,100f,300f,500f), desc,style);


        if(GUI.Button(new Rect(700f, 290f, 150f,50f), "Cancel")) {
            currentItem = null;

        }


        if(GUI.Button(new Rect(700f, 230f, 150f,50f), "Use")) {
            currentItem.Use ();
        }
    }

    //1st column
    GUILayout.BeginArea (new Rect (80f,60f,600f,600f));

    for (int i = 0; i<5; i++) {
        if(items[i]!=null){

            if(GUILayout.Button (items [i].texture, GUILayout.Width (80f), GUILayout.Height (80f)) ){
                currentItem = items[i];
            }


        } else {
            GUILayout.Box("", GUILayout.Width(80f),GUILayout.Height(80f));
        }
    }

    GUILayout.EndArea ();

    //2nd column
    GUILayout.BeginAr...
}

Copyright Notice：Content Author:「Rinomancer」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/30768232/press-one-button-focus-all-buttons