In our lives, we have many relationships. Some are successful, some fail and go horribly. But we keep trying till we meet "the one". You know, our soul mate. The person we are 'destined' to be with. To be honest, I am still on the look for my soul mate, but it appears that Ariana's search has ended. These lyrics are showing that she has found the love of her life. "But I knew you were the real thing when you walked through that door" is a lyric that shows that for her, it was love at first sight. And she is saying that her actions make it obvious that she is completely in love with this person.