Imagine that there's a cookie sitting on a counter for a week and you somehow didn't notice it and eat it until Friday. Now imagine that you can go back in time and that you do go back in time to earlier in the week. The cookie's still there, but if you eat it now, then how in the world was it there for you to eat on Friday before you traveled to the past? There's no telling what could happen if you ate it because that would mess with reality as we know it.
Now switch that example around so that you're Link and the cookie's a piece of heart and you're good.