I've often heard of people who wonder why God put the tree of the knowledge of good and evil in the garden. Was He just looking for an excuse to kick Adam and Eve out?
Short answer: Free-Will.
Longer answer: God wished to give humans free-will, or the choice to follow Him or not, as a result of this He was required to make a rule that we could at least theoretically disobey. God in His infinite wisdom¹ decided that the tree of the knowledge of good and evil would suit this purpose ideally, so He made it and told Adam and Eve not to eat of it.
1. Whenever you're unsure why God did something (in this case chose the tree), just say "God in his infinite wisdom..." and no one even realises that you basically just said you had no idea. :-)
