Why is Goku always hungry?

Goku is a Saiyan, but as a baby when he was send to earth to destroy it.. he felt and hit his head. Radditz came back to finish the job and also to find him. After encountering some of the other Z fighters, he senced a great power level, being Goku.

Vegeta and the bald one came for the dragon balls en to kill Goku. They where aware of the balls thanks to the communicator on radditz eye, wich recorded the Z fighters talking about them.