[Dr. Sivana]

But, why does everyone look so happy?

My understanding is that early cameras needed to keep the shutters open for a while. Using color filters possibly made it worse. It is hard to hold a smile perfectly still for a couple of minutes.

[AFreeBird]

The second photo, next to the creek, gives a good indication of the long shutter speed based on the look of the water.

But damn amazing photos for being a century old.

[frog in a pot]

But, why does everyone look so happy?

Good question.

At first I suspected it was because in 1910 folks from that culture may not have been that comfortable around cameras, if they even knew what a camera was.

But Doc nailed it. From the comments:

Incidentally, very few people are smiling in old photographs because the exposure times might be from 15 seconds to several minutes, depending on the emulsion in use, light, and lens. Hard enough to hold still, much less to maintain a smile!

The lack of smile is due to the technic used by the potographer: he had to expose 3 different plates to get a color shot so the subject was exposed for a long period of time and should remain still so smiling was almost impossible. You can see in the picture involving a river that the exposure was quite long enough to give that smooth effect to the water.