The Sun and the Sunflower

Everyone is born with a birthmark tattoo.
When you fall in love with someone, their birthmark appears on your body in the same place it appears on theirs, and it never goes away. Not even if you fall out of love or if they don't love you back.
Zack falls in love easily and has the birthmarks to show for it. Jack is cautious, but that doesn't stop Zack from trying.

Based on this post.