I imagine our hearts like a treasure box with a treasure in it. Some people have unlocked it and given their treasure away only to never get it back, and so they get jaded, leave their gilded box to rust and disrepair. I feel like if you keep tending to that little box you'll get your treasure back, but it won't shine as much as before, or the some of little inlaid diamonds have fallen off in all the tussling.
Then there are those who have a beautiful treasure box, golden and gleaming, polished everyday, and when you open it it's spotless on the inside as well, but it's empty. There's no treasure, and you feel like this extraordinary treasure box is left a little wanting, because it isn't being used to contain something even more extraordinary. The box is waiting, you see, and so are their owners. Waiting for someone with a box just as beautiful and brilliant as theirs to come along, and put something even more beautiful in their box. To give them back their hearts.
There are people who are broken, and others simply lonely, but that doesn't mean the box can't be repaired and polished again, or filled up with a brilliant treasure.