In one of the best books i have recently read, Antifragile, the author Nassim Nicholas Taleb, talks about the concept of via negativa. Basically, Taleb says that you can improve your life by subtracting, not adding more things to your life. He states several examples in life, like in health, in medicine, and of course, in finance(most concepts in the books are explained through finance).
Via Negativa applied to happiness( and sadness)
Via Negativa can also be applied to happiness and sadness. Most people go through life collecting, acquiring, loaning out more and more things, with the expectation that this time, they will truly become happy. But of course, that never happens. Most of the happiness gotten by acquiring things is temporary, and the bigger the upswing, the faster the downswing. Most people buy something, get excited about it, and then go back to their normal state. Quite often, for temporary happiness, they give up things permanently, like money, time and relationships.
In contrast, the things that give a permanent boost to your happiness are all gotten by subtraction. The happiness you get by being healthier is often obtained by removing calories out of your daily intake. The calm and comfort you feel after seeing an organised room is gotten by removing things you don't need.
The opposite applies to reducing sadness
People who are sad, or depressed or just in general in a bad place in life, often reduce contact with their friends and family. They often reduce their food intake and their sleep. Of course, in most cases, this only makes the problem worse..
I think that the way to reduce sadness is to do the opposite. Its to add things to your life. If you are sad or depressed, adding a morning walk in a park, or listening to calm music, will do wonders to your mind. Adding exercise will also help immensely.
A caveat
Any addition or subtraction you make to your life must be consciously made.
If you become over-confident and stop thinking critically and rationally, it definitely won't improve your happiness in your life. In the same way, if you are depressed and try to solve your depression by adding a food or drug addiction, its not going to make your life better
Any change you make in your life has to be consciously made. If you just start or stop doing things while drifting in life, it's never going to solve problems or improve happiness (In fact, it probably will do the exact opposite).