Pro 25:16 Hast thou found honey? eat so much as is sufficient for thee, lest thou be filled therewith, and vomit it.
Pro 25:17 Withdraw thy foot from thy neighbour's house; lest he be weary of thee, and so hate thee.
Pro 25:18 A man that beareth false witness against his neighbour is a maul, and a sword, and a sharp arrow.
Pro 25:19 Confidence in an unfaithful man in time of trouble is like a broken tooth, and a foot out of joint.
Pro 25:20 As he that taketh away a garment in cold weather, and as vinegar upon nitre, so is he that singeth songs to an heavy heart.
Pro 25:21 If thine enemy be hungry, give him bread to eat; and if he be thirsty, give him water to drink:
Pro 25:22 For thou shalt heap coals of fire upon his head, and the LORD shall reward thee.
We should be good to others out of goodness sake, of course. But if you need a bit more reason, verses 21 and 22 tell you yet another reason to be good to your enemies.