Why, the default context in a function that hooked to an object is not itself

While going through Javascript basics, I learnt that this will always be the object which is before the period operator, but

          var ob = {
              x:"",
              setx: function(){
                x = "got u"
                // in this function this is nothing but ob
              }

          }  
         ob.setx();
         console.log(ob.x); // this is giving me still ""
         console.log(x); // getting here as 'got u'

this in setx should have context of object ob, but why it's accessing the global space instead of accessing it's own x.

If I use setx as shown below it works

               setx : function(){
                 this.x = "got u"
               }

Why we have to use this keyword, even though 'getx' context is bind to the ob Object

Am I missing any basics?

Is there any design pattern where we can share data(should be private to the parent) between child functions? Such that we can reuse the child functions as we go.