The sweetness comes from the lactose (disaccharides)… this natural sweetness in milk is accentuated with heat, as the milk is heated the natural sugars are more obvious to the senses.
Now heat is another discussion... Suffice to say that if you over heat the milk, you'll ruin the texture of the milk. There is a key temperature for this : it is 65 degrees Celsius.
Anything above this makes the milk split into a dry foam and boiled milk.
The last factor is protein. Protein comes in simple terms as grass, and specifically grasses feeding. Cows fed on a high grass diet will produce higher protein milk. This is essential for the milk texture. As you heat the milk you release the proteins. Again too hot and the milk splits and loses its texture. For really great milk try different varieties.
The most important thing is to find what you like. The sensory taste and texture is really important when making coffee. If you can deliver this texture and mouth feel regardless of latte art and all that fancy stuff, you will have great tasting coffee.
