That's like saying dogs evolved from cats. We did not evolve from monkeys. We probably evolved from another primate ancestor of monkeys and they evolved from a lower sub-species of primate. You have o understand that monkeys have evolved as well.
Both monkeys and apes shared a common ancestor, around 40 million years ago they began to split off into the various branches and then split even more into all the sub species we have today. Look at it this way, the other apes are our brothers and monkeys are our cousins. Here's more detailed information. www.nas.edu/evolution/
Just to be clear, what you are asking has nothing to do the Darwinian Theory of Evolution. There are already some who have given you an appropriate, science-based answer to your question. Check out the link provided by Caluvox.