Why doesn't cottage cheese melt? Most kinds of cheese melt but cottage cheese does not at usual cooking temperatures. 

During the preparation of various kinds of cheese, I have heard of a coagulant named ‘rennett’ being added along with the acidifying agent. For cottage cheese it is just lemon juice mostly (acidifying agent). Does this have anything to do with the difference in properties?
You essentially make cottage cheese in the process of making cheddar cheese. But cottage cheese is about 80% water. The "cheese" solids are soluble in oil, but not in water. So cheddar cheese will "melt" but cottage cheese won't.