What do you notice about most statues? That is to say, aside from the fact that they're usually huge, typically only one color, and made from some pretty solid material like marble or iron. And let's not have any discussion of the tastefully understated genitals on nude sculptures. What I tend to notice is the dour, almost disapproving glances that the faces of most statues have. Some don't even have this touch of personality; sculptures like Michelangelo's David have these vacuous stares that make you wonder what conceivable emotion the subjects could be feeling. Most people who have statues made in their honor are supposed to be taken seriously. Look at a bust of George Washington or Julius Caesar and tell me that the expression you see isn't meant to convey a sense of gravitas, as if the subject is someone who is supposed to make you think "what a man!" by his very icy stare.
This isn't how sculptures used to look. For most followers of Western Civilization, the ancient Greeks are considered the founders and best makers of realistic sculptures. During the couple of centuries before what we consider classical Greece, Greek art was sophisticated but lacking in naturalism. Art is supposed to represent some intangible ideal and throughout the ages, this ideal has tended to become increasingly more realistic. Archaic sculptures in general are not particularly dynamic; the subjects are typically in static, uninteresting poses with stiff limbs and almost impossibly straight postures. Looking at Greek sculptures from about the eighth century BC up until the fifth, all of these traits are present in addition to one other almost constant feature: a conspicuous, unsubtle smile. This ubiquitous grin is known as the archaic smile.
All of these motifs likely inherited from the ancient Egyptians, who were the original makers of great monuments. This statue of Ramesses II has them as does this iconic one of Akhenaten. Notice that the smiles here are somewhat wry and not overstated. Probably they're designed to show that the Pharaohs are happy and in generally good spirits. These statues date from the 13th and 14th centuries BC, respectively. This notion seems to have been transferred to the Greeks in an exaggerated fashion. One of the most famous exemplars of the Greek archaic smile is the Krosios Kouros; note that all male sculptures from this period are depicted as being in the nude. The female equivalent would be the Peplos Kore; female sculptures are always tastefully draped. Look at Athena here - she's happy as a clam. And check out this dude...what a mug! These guys are just glad to be alive, or at least the closest thing to alive that inanimate objects made out of stone can be.
There is some speculation that the archaic smile was meant to convey a sense of life to otherwise lifeless things. Rocks are fairly stoic and they just sit around; living people smile and have a good time. Eventually, however, the artistic standard became rather divorced from the concept behind it. Here's one of the most famous examples of the archaic smile being taken to its logical extreme. This is a fallen warrior pulling a spear or arrow out of his chest. This statue represents a man dying what is most likely a horrible, agonizing death, being pierced in the liver and all. And yet, there he is, all smiles. The archaic smile ceases to appear as a prominent feature of Greek art after about 450 BC and most facial expressions are either grim or just vapid. What the smiles actually meant (if indeed they meant anything) are debatable. For centuries after the Greeks gave it up, sculptures from the Far East in places like Korea and Indochina regularly featured smiling faces, with Buddha statues being the most common of this type.