I think the story goes like this:
He met a girl and connected with her. He was drawn in by her looks, but as they became friends and he got to know her, he realized she was more than just a pretty face. He ends up falling for her, and is singing this song to her, to tell her how he feels. When he says he'll be sure she's a friend, he's hinting to her that he would choose to fall in love with a friend just like her. He knows she's the right one because they're close and he trusts her, and he wants her to know that and to be with him and share his love.
I think the story goes like this: He met a girl and connected with her. He was drawn in by her looks, but as they became friends and he got to know her, he realized she was more than just a pretty face. He ends up falling for her, and is singing this song to her, to tell her how he feels. When he says he'll be sure she's a friend, he's hinting to her that he would choose to fall in love with a friend just like her. He knows she's the right one because they're close and he trusts her, and he wants her to know that and to be with him and share his love.
It's an incredibly beautiful profession of love.