Relationships matter -- a lot.

"At 56, for people who put energy into families, there was a big payback," said Michael Apted.

Again and again, the "Up" series reveals how critical friends, family, and social connections are to an individual’s success and happiness in life. For both men and women, caring for others invested their lives with a meaning that they weren't able to find in any other activity.

Money also matters -- but only up to a point.

The series originally set out to examine the long-term impact of social class on happiness and life chances. And it does, in fact, show that money matters. But the research also shows that once we secure food, shelter, and a minimum level of comfort, happiness springs from life meaning and relationships much more than money.

Life goes on.

Through the years, we've seen the participants experience deaths, divorce, and dramatic career setbacks. We've also seen them prevail against adversity.

Over and over, we've seen the participants believe negative events meant their lives were finished. But people will gradually return to their personal "baseline" of happiness after a triumph or a tragedy.

The key to happiness is to raise that baseline by cultivating an approach to life that emphasizes appreciation for the good things, as well as qualities like forgiveness and compassion.

Count your blessings.

The most troubled -- and compelling -- participants in the series are the ones that are also among the most grateful for what they have.

Neil, for example, suffers severe mental illness and has been homeless and suicidal for long stretches of time. But Neil also provides the audience with many moments of grace, as when he recounts how grateful he is for long walks and conversations with friends.

This sense of gratitude contributes powerfully to the resilience self-cure of Neil. People who live a life of pervasive thankfulness really do experience life differently than people who cheat themselves out of life by not feeling grateful.

Don't compare yourself to others.

Yes, research does find that inequality makes us unhappy -- but it turns out that this has little to do with absolute amounts of money. It's the comparisons that hurt.

We hurt ourselves with comparisons to others, but we can't seem to stop. Remember that each life must be judged on its own terms.