Our ability to maintain relatively constant internal conditions is due to homeostasis. It may sound pretty boring, but without homeostasis we would have to change the way we do a lot of things. For example, without the ability to regulate our body temperature we'd have to rely on external sources of heat to warm us (think of a lizard sitting on a rock in the sun...not very productive, right?). Homeostasis is also important for regulation blood glucose levels, which I'll explain in a little more detail.
So, how does our body regulate our blood glucose levels to maintain homeostasis? The internal mechanism for blood glucose regulation is negative feedback. As you can see from the figure, depending on whether glucose levels are rising or falling, the body has a different response. When levels increase, the beta cells secrete insulin which then converts glucose to glycogen so that extra glucose can be stored restoring glucose levels to a normal level. When levels fall, the alpha cells secrete glucagon which converts stored glycogen to glucose increasing levels back to normal.