Read John 3:29 Using Other Translations
He that hath the bride is the bridegroom: but the friend of the bridegroom, which standeth and heareth him, rejoiceth greatly because of the bridegroom's voice: this my joy therefore is fulfilled.
The one who has the bride is the bridegroom. The friend of the bridegroom, who stands and hears him, rejoices greatly at the bridegroom's voice. Therefore this joy of mine is now complete.
It is the bridegroom who marries the bride, and the best man is simply glad to stand with him and hear his vows. Therefore, I am filled with joy at his success.