"'Again, you have heard that it was said to the people long ago, "Do not break your oath, but fulfill to the Lord the vows you have made." But I tell you, do not swear an oath at all: either by heaven, for it is God's throne; or by the earth, for it is his footstool; or by Jerusalem, for it is the city of the Great King. And do not swear by your head, for you cannot make even one hair black or white. All you need to say is simply "Yes" or "No"; anything beyond this comes from the evil one.'" NIV translation
We see people make oaths all of the time but, in the kingdom way, it is not necessary. If you always tell the truth and do what you say, then, there is no need for any other words. He goes so far as to say that any other words are from the devil. Why? Because, when we swear by anything, we are asserting that we are in control of those things when in fact God is the only one in control. Therefore, it is as if we are elevating ourselves to the position of God which is what Satan tried to do.