Before he was Galactus, the world eater, Galactus was just a normal humanoid alien. This was all before the Big Bang. I know a lot of you Stephen Hawking fans are thinking, "what do you mean BEFORE the Big Bang?" This is comic book land, people. Stick with me. Before the Big Bang, everything was sucking up into one little egg of matter, including his planet. As it sucked everything in, the Phoenix Force (the same one that inhabited Jean Grey in X-Men) resurrected him as the Big Bang happened. Thus, Galactus was formed.
What does a creature reborn out of the Big Bang eat? Planets. Makes sense. At first he was pretty chill about eating planets. He'd only eat the ones that didn't have any life on them, but after a while you start to run out of places to eat. That's when your morals go out the window and you start chomping on life-infested planets.