Yes. Sources suggest that if someone says 'you stole my heart', this means that you now have a lot of influence on that person’s life. The person didn't actively seek to fall in love, but that's what happened.
Sometimes it makes the person weak because it has taken the persons value system right along with it. It's not that the person doesn't see right from wrong; it's just that he/she is so lost (all sense of character and strength) in what you give him and his feelings for you.
Relationships get better or worse depending on how much or how little we understand and invest in them. Check out some tips on how to keep a healthy relationship strong at Helpguide.org.