While the inevitable passage of time certainly takes a lot of things away from us, it does provide quite a few as well. One of its most important contributions is to the way we feel.

Positive and negative affects (mood) seem to operate independently of each other: you can have a lot or a little of either or both.

"High affect" people score above average on both positive and negative mood levels, while "low affect" people score less on both.

No matter which type you start out with, you can expect your affect levels to change in a favorable trajectory over the course of your life.

According to research published in the Journal of Personality and Social Psychology, both men and women see an increase in positive affect as they get older (women, in particular, see a greater and accelerating increase).

On average, negative affect does not change for women predictably with age, but it decreases for aging men (with the caveat that the decrease is more pronounced for married men; for unmarried men, negative affect is elevated at every stage of life).

Many theories have been proposed for why this happens. But a 2013 review of research indicates that older people develop at least three distinct emotional skills: 1) they react less to negative situations, 2) they are better at ignoring irrelevant negative stimuli than they were when younger, and 3) they remember more positive than negative information.

Basically, it's like knowing your negative emotions won’t last so you disregard them when they do arise and get a head start on the good feelings.

Arthur Brooks, who is a professor at the Harvard Kennedy School and the Harvard Business School, where he teaches courses on leadership and happiness, thinks that when we're talking about growing old, we also have to mention changes in personality.

"Personality is generally separated by psychologists into five parts: openness to experience, conscientiousness, extroversion, agreeableness, and neuroticism," Brooks writes, discussing a paper by two researchers in which they summarized ... a voluminous literature on how these dimensions of personality change as we move from childhood to old age.

"Some of the changes are not necessarily either good or bad. For example, people tend to become less gregarious after their mid-50s; they become more assertive from adolescence through their mid-30s and stay at this higher level; their openness to experience rises into early adulthood, stabilizes, and falls after their mid-50s."

But other changes are unambiguously positive. "Agreeableness tends to rise throughout adulthood, probably as we see its benefits and establish more emotional equanimity," Brooks adds. "Neuroticism usually falls, at least until one’s late 60s. And conscientiousness rises continuously. If you follow the typical development, you can expect to be nicer and kinder, and less depressed and anxious, when you are old."