The Theory Of Pokemon Evolution
Note: This is only a theory and has not been proven true or false.
Some pokemon evolve by levels. Some by stones. Some by happiness. Some not at all. My theory is that pokemon evolve by the amount of energy inside of them. When the energy has increased enough, they may evolve. Different things increase energy within a pokemon.
The most astounding of all is the pokemon Eevee. It evolves by happiness or stones. Both increase the amount of energy inside. Stones to Jolteon, Flareon, or Vaporeon. Happiness during day creates Espeon. Happiness at night forms Umbreon. However, there is a new study being conducted to find other ways of getting Espeon and Umbreon.
Deoxys doesn't evolve, but Energy plays a big role in its power. It can increase or decrease its energy at will (hence the regeneration). This also allows it to change forms. There are rumors going around that Deoxys works everyday to try and create a new form.