Whenever you make a post, do you notice that six-pointed star at the bottom left-hand corner of the post? That's the rep button. You can click it and give the user reputation. More rep means more hearts. If you click the star on your post, it will tell you if you got rep for it or not.
Consequently, you and other users can give out bad rep, which decreases hearts. I'm sure you've seen users with skulls instead of hearts. This means they have negative rep.
Personally, I wouldn't worry about rep. I'm not all crazy for it, partially because it's often abused and is just a big popularity game. I don't need 8-bit hearts to tell how cool I am. This is why I keep my rep hidden.