When someone receives the key to the city, it is given to someone who has done something to earn the right to have it.   The public official will not present it to someone who is not trustworthy.  When you give someone the key to your heart, you should follow the same principle.  The heart is a delicate place, and very vulnerable for someone to cause hurt.  But someone who is trustworthy, someone who has earned the right to be there, will take great care in not harming, but care in healing your heart.