Galactus was born when the previous universe "crunched" and produced the Big Bang. He gestated in a kind of egg until an intergalactic war woke him up. His long hibernation had produced a hunger so great, he devoured both sides. From the remains of the planetary system, he created his worldship, and began traveling the universe searching for worlds to consume.
Eventually, his travels led him to the world of Zenn-La, where Norrin Radd offered himself to act as his herald, to search out appropriate planets for Galactus, if Galactus would spare his world. Galactus agreed, and since then, has always had a herald working with him.
|A herald is born...|
Of course, Stan and Jack's immortal Fantastic Four 48-50 brought Galactus to our world, and Earth was now one of a select few worlds that stayed his hand. Galactus has continued to devour worlds since then, and is a source of terror to all sentient beings.
|Devourer of Worlds|
By the way, to all of you visitors to our site, we hope that you will play along too. Just write your best one-pager of script in the reply section right under this Why. And everyone can share that appetite for destruction...
|...my god, it's full of beans...|