This statement reflects the enduring presence of someone in the speaker's thoughts and heart. "I Realized I Was Thinking Of You" signifies a sudden awareness of their continuous preoccupation.
"I Began To Wonder How Long You'd Been On My Mind" indicates that the speaker is contemplating the duration of their thoughts about this person, suggesting that it has been a significant amount of time.
The phrase "Since I Met You, You've Never Left" beautifully captures the idea that the person addressed has had a permanent and lasting impact on the speaker's life. It signifies an enduring connection and an unceasing presence in their thoughts and emotions.