Have you ever wondered why some people always seem to be happy? This led me to the realization that each of us has our own baseline level of happiness. On a scale of 1 to 10, where 1 is "there is no reason to live" and 10 is the most ecstatic you have ever been, I'd say the average of my days would be something like a 5.5 with a standard deviation of 2 (meaning most of my days are between a 3.5 and a 7.5). Two standard deviations would result in a low of 1.5 and a high of 9.5, which are much rarer. A 3.5 might be a terrible day at work, and a 7.5 might be a relaxing day on the water. You can fill in the rest.
The two variables, baseline and standard deviation, are different for everyone, and in my mind, both of these are able to increase or decrease over time to a certain extent. While nature and family history may anchor your baseline, nurture and upbringing can influence it above or below this innate level. The subsequent component of the average variation in perceived happiness is also able to be influenced positively or negatively. To my understanding, antidepressant medications can reduce the standard deviation of happiness one experiences. This means that you may experience fewer bad days but may, conversely, not be able to fully experience the most positive days. On the other hand, I sometimes think that meditation, yoga, and religious practices can increase the standard deviation of happiness. This is due to these experiences making you more self-aware through broadening the possible emotions that you experience. If these practices do not lead to higher highs and lower lows, they at least have the tendency to make you more at peace with your baseline level of happiness.
Dogs are fascinating to consider in this context, as they tend to be happy almost all the time. A dog's measure of happiness may be on a scale that is shifted to the right a few points when compared to humans, such as a 3 to a 12. This means that their average baseline happiness may be as high as a 7 compared to that of a human. They experience a 3 when they're left at the pound or at the kennel for the weekend and think that their owner is gone forever. Conversely, they may experience a 12 every day when you get home from work and open the door to greet them. Sometimes they are so excited that they even pee on the floor. Have you ever done that? Neither have I, and perhaps that's because we've never experienced what a 12 is like.