Feeling sad is a natural occurrence for all human beings. Even the animals do feel sad whenever they loose their loved ones ones. But for humans, there are various conditions that causes sadness. We can't be 100% happy everyday, feeling sad is a sign of human hood.
Based on my research, I was able to get some factors causing human sadness. But I will only speak on a "very common" one which is the concept of "Self comparison".
yH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==.jpgimages(239).jpg
Most people of today are always sad because they compare their life with others. They compete for materialistic possession by imitating others. A good example can be sighted on 
Are you the kind of person who feel sad because your wallet is empty anytime you post reasonable articles or blog? The depression increases when you see other steemites gaining more money. This happened because you never believed in yourself, you kept comparing yourself with others. I will round up by saying happiness is a choice. You either choose to be happy or you choose to be sad.
My fellow steemites, please avoid comparing yourself with others "because it is a big insult on your personality just as Bill Gate said"
Feel free to comment and ask questions.
For more enticing and motivational post follow me 
Thank you for reading.