Happiness can be explained and measured using various different scales, for instance through subjective experience or the levels of neurotransmitters typically associated with positive feelings like  serotonin, dopamine, or oxytocin. Although the ways of measuring it seem relatively straightfoward, the question of what is happiness, and how one can attain it, are a little more complicated.
Achieving or sustaining happiness is a question of cognition, conditioning, and context. What this means is that happinesss is a function of how you look at what you experience and which neural connections your current experience triggers. Studies undertaken by Dr. Barbara L. Frederickson provide evidence that how you generally feel, your emotional state, is actually similar to ecological states.
This means that mutually-exclusive states (like happiness and depression) lay on different sides of a tipping-point, in this context meaning that a certain ratio of positive to negative emotions. These emotions, which you feel daily, collectively decide your average levels of neurotransmitters as well as your subjective experience over time. Like ecological states, this state maintains a certain level of resiliance, or resistance, towards change. It means there are tipping points: past a certain ratio of positive:negative emotions then you will feel happy, and under this ratio you will very likely experience the world with less glee.
Happiness is not just a decision: it is a state.
To test this, a series of studies were undertaken in which test subjects would meditate on positive emotions (primarily love) daily, and during which the control groups did not. As Frederickson says in her book “Positivity”:
“On average, people didn’t report any reliable increases in their positive emotions until the third week of meditation practice. Even so, positivity continued to rise steadily throughout the study… it’s distinct from what you’d find from placebo effects, which tend to be large and immediate… it tells me that even small shifts in our daily diets of positivity -if enduring- can change our lives for the better” (p. 87)
 happywoman
This provides evidence that how you experience your time in the world has a significant effect on how you feel overall. Surprisingly, the study showed no decrease in peoples’ reports of negativity: the people were not happier because they avoided recognizing or experiencing negativity, but because they added sufficient positivity to their lives.
This means that it isn’t just how we look at things that affects our overall experience, but also the ratio of positive to negative experiences we have. Frederickson’s study found a ratio of 3:1 was the key to happiness, the key to increasing emotional resilience towards situations that would otherwise have been overwhelming.  It means we can boost how we feel by conciously adding enough positive situations in our daily lives: by meditation about times we felt good or loved, by deciding to appreciate the little things in the world around you.
As everyone who has ever experienced anything traumatic knows: it isn’t always possible to just “decide to be happy.” Of course, how we decide to see the things that happen to us, what thoughts we decide to indulge in or not, how we decide to see ourselves, has a major impact on how we feel, especially in the long run. But, some experiences are truly undeniably negative and it is offensive for those of us who have a higher ratio of positive to negative emotional experiences, to tell these people that it is all their fault they feel badly will likely just make them feel worse and be a further detriment to their self-perception.
The way to help people who are down, the way to help yourself, is to recognize that happiness is a distillation of many different parts of your life. That isn’t saying we shouldn’t always look for the silver lining, shouldn’t use delibrate effort to appreciate what we have and the good things we come into contact with, that we shouldn’t meditate about times we felt good: it means we have to also be concious of our emotional ratios and work to improve the number of positive experiences we have.
It means that the best way to help someone who is depressed is not to tell them to “think happy thoughts” or that their life is fine, and that they have to decide to see it differently, but to help provide them with positive experiences and to teach them about how they can use these memories, use the occasional positive thing that  does happen to them, to improve their overall mood and health in the long run.