The mood is likely optimistic and full of hope/happiness: the man is happy that he is a man. He is optimistic in wishing to be the sun to brighten someone’s day, or to light someone’s way- these are helpful qualities. The mood is definitely positive here- like a man saying this to his friend or son. He is acting as a guide. I might even say romantic- like a man saying this to his love; meaning he will brighten up her day when she is sad, or guide her when she is lost. (This can also interpreted as a father saying this to his son, or vice versa, and/or a friend saying this to another friend.) Hope this helps 🙂