Is the difference an evolutionary thing and once upon a time we smiled for the same reason wolves smile today-?
Wolves and other canids don't "smile"; they draw back their lips to protect them from damage. Humans withdraw their lips for the same reason, but a relaxed expression of pleasure on the face also causes the lips to part. I suspect that since the Bible mentions God smiling (Job 10:3), rejoicing (Isaiah 62:5) and delighting (Psalm 147:11), the difference appears to be due to the fact that we as humans are made in the image of God (Genesis 1:27).