God wants his creations to serve Him willingly. He's loves his creations. His creations cannot love back unless they have the choice to love back.
In order for one to be capable of love, one must be capable of the opposite. That is, one must have free will.
Now, having free will includes the capacity to turn from God. One cannot exist without the other. It's cause and effect. Now, this can and does lead to many evils (just look at our world).
Therefore, a loving God must allow for evil to exist in order for love to exist.