I'm pretty emotionally stable. I don't have very high highs or low lows. My peaks and troughs don't last very long. Overall, I am a happy person. It's hard for me to get to such great heights. For instance, I say that I don't have fun very often. Some people disagree with me, but I define fun as a sensation of being caught up so much in the moment, enjoying every moment of what you're doing that you forget about everything else. I also say that I don't miss people (very often). Some friends beg to differ, but I guess instead of associating "miss" with "fond memory," I associate it with "long for their company."
Likewise, I rarely get mopey. If I am down, it usually lasts for less than a few hours. In fact, it's always a new day tomorrow and tomorrow always starts fresh. It's kind of interesting how certain people can affect my mood more than others. Today, one of those special people had me excited to see her, then I didn't get to see her and I was down for about an hour. I think this builds on my expectations hypothesis: don't expect too much and it's harder to get let down. I think now that it's better to be pleasantly surprised than a little bit disappointed.
I'm not saying that you should be a pessimist. I like to think of myself as a realist. Set realistic goals and achieve them. If you lower your standards a little, then you can reach your goals more easily...but each accomplishment isn't worth as much. On the flip side, lofty expectations can be unattainable (especially if they involve other people who can drop the ball along the way) and failing to reach them can leave you down.