There is an old saying among wedding photographers, "If the bride and groom (and their families) are happy, your job is pretty much done."
Now, any experienced wedding photographer will tell you there is much more to creating wonderful wedding memories than just a happy couple, but the emotional state of your subject (and your emotional state) has a serious impact on the image. This applies to every type of photography I can think of, not just wedding photos.
If I show you a photo of a crying child in a Santa hat your instinctual response will be to feel sorry for the child. If I show you a laughing baby in a Santa hat your response is likely to be happiness and joy ... the kind of feelings people want to have around Christmas time. The simple truth is that human beings are emotional creatures. If you show me a portrait of someone who looks uncomfortable I am going to feel uncomfortable about the photo. More importantly, if you show me a photo that lacks any emotion, I'm not going to feel anything about that photo.
That does not mean that every photo needs to be of a living human being who is expressing emotion, it means you have to bring out the emotion in your images and the people who view your images. Some of the greatest photographers in history made their careers by bringing emotion to objects that that have none ... and making people feel something when they look at the photos. Ansel Adams is not famous for taking snapshots of mountains but for making people feel the depth and majesty of unique landscapes. Bruce Gilden became one of the Magnum Agency's best photographers not because he took photos of weird people and objects ... he makes people feel regardless of whether he's taking a portrait of a homeless man or crafting an image of empty boots for a fashion magazine.
Photography, like any other art, is only powerful when it evokes an emotional response from the viewer. Whether it's a smile, a feeling of sorrow, or a sense of awe and wonder, emotion is the difference between an image that lands on the cover of a magazine and one that ends up as a sidebar on page 60. Feelings are what separate unforgettable memories and snapshots.
Feel free to share your stories about how emotions made (or didn't make) an image into something truly special.