Quick: which operating system do you think is happier, Windows or OS X? Even if you've already formed an opinion, you might still be wondering, "What kind of inane question is that?" After all, how could software possibly be happy? This is nonetheless one of the reactions I get when using products that are designed by Apple in Cupertino. It's not specifically that the product is making me happy as much as I have this vague feeling that the product itself is somehow happy. The otherwise rational engineer in me knows this is silly, but it is perhaps nonetheless irrationally true.
If visual design speaks to the user's aesthetics, and interactive design to the user's cognition, then this seems to be something else. Aside from the notable exception of Don Norman's Emotional Design, this is an aspect of design that we don't often think about: playing to the user's awareness of emotion.
As humans we have a tendency to anthropomorphize inanimate objects, particularly ones we spend a lot of time with. Perhaps any readers with a background in psychology can provide theories for why we exhibit this irrational behavior in the comments below. But regardless of why, it's something we as a species do. We name our cars, we attribute gender to things that have none, and we think electrical outlets have faces (they look sad). Even objects incredibly far from the depths of the uncanny valley can be a bit anthropomorphic—things like a file manager, or even a desk lamp.
In its very early days, Pixar put a lot of effort into trying to understand what properties made something appear alive. They spent a considerable amount of time intently studying the way animals and humans moved, and they quickly picked up a few basic properties. For instance, there are a lot of accelerations and decelerations in biological movement. This is true even if we are trying as hard as we can to move only at a constant velocity (e.g., when performing a very serious dance commonly referred to as "doing the robot"). If you take these properties of biological motion and apply them when animating a simple inanimate test object—say, a basic desk lamp—it magically becomes alive once you introduce acceleration and deceleration.
When the lamp looks up at you, it is suddenly looking at you; the bulb is an eye, the shade a head. If you get the lamp to jump up and down a bit, the next thing you know it's the happiest lamp you've ever encountered:
But it isn't just lamps. Why throw up a boring dialog box in the center of the screen when you can get capture the user's attention with a jumping icon? The icon doesn't have much to say ("replace the file on the server?") and it looks like a tiny truck, but nonetheless the way it moves reminds us of the way a dog jumps up at you, drooling and completely happy.
Another aspect of biological motion is that we tend to decelerate before we collide with another object. This of course makes sense as we can't move at a constant velocity even when we try to dance like a robot, and running into stuff at high velocities hurts, so it's really best to slow down first.
This is one of the things that struck me about the design of the iPhone OS scrolling model. It accelerates and decelerates, and if you pull something too far away, it is sure to slow down as much as possible before the collision. And while the iPhone and iPad application icons don't bounce, they do have a sort of wiggle dance when you move them around.
These are all just animation primitives and simple physics models. When applied to software they are subtle, they don't go nearly as far as to create a happy and curious lamp. But they manifest in ways that make products simply easier for us as emotional irrational humans to love (or at least makes it easier to occasionally forgive them).
Animation in software can serve a lot of important purposes. Animation can draw the user's attention, help the user build a mental model of navigation events, and indicate changes in state. But beyond all of those very functional ways that UX designers can employ animation in their work, it can also impact users on an emotional level: it can make your product seem happy, and ever so slightly alive.
 By this I mean the North American 15 A/125 V grounded outlet looks sad. Just one tiny change in the initial design to the grounding socket and the entire continent could have been filled with happy outlets.
Pixar Lamp Photo by Matthew Cachia