God is the only One who could swear to Himself because He has never broken a promise.
When God Swears by God
Since he had no one greater by whom to swear, he swore by himself, saying, “Surely I will bless you and multiply you.”(Hebrews 6:13-14)
There is one Person whose worth and honor and dignity and preciousness and greatness and beauty and reputation is more than all other values combined — ten thousand times more, namely, God himself. So God swears by himself.
If he could have gone higher, he would have gone higher. Why? To give you strong encouragement in your hope. What God is saying in swearing by himself is that it is as unlikely that he will break his word of promise to bless us as it is that he will despise himself.
God is the greatest value in the universe. There is nothing more valuable or wonderful than God. So God swears by God. And in doing that he says: “I mean for you to have as much confidence in me as it is possible to have.” For if more were possible, verse 13 says he would have given us that.
Now this is our God. The God who is reaching as high as he can reach to inspire your unshakable hope. So flee to God for refuge. Turn from all the superficial, self-defeating hopes of the world and put your hope in God. There is nothing and no one like God as a Refuge and a Rock of hope.