I love doing this for people! My mom did this for us growing up on our bedroom doors, and I even incorporated it into my teaching by having each of my classes choose someone in the school to be heart attacked. They'd write that person a small note on the heart, and we'd sneak to that person's office or classroom door, stick them all over the door/wall, and scamper away giggling because we felt like we got away with something. It's so much fun, AND it teaches kids to do something nice for someone else "just because".
As we were taping the hearts on our friends' door this morning, Connar said to me, "I love doing this for _____ , Mommy." I asked him if it made him happy to make _____ happy, and he said, "Yes" with a big 3 year old smile. Now that made my heart smile. :)
You don't have to go all out like this (although it really didn't take too much work and was so much fun for the boys to color the hearts...older kids and adults can write words or a personal message on the hearts), but I encourage you to do something nice for someone else "just because". I promise it will make your heart smile too. :)