It might be related to the human nature. Recent studies have shown that most of our thoughts have a negative connotation. I believe it has to do with basic instincts such as survival. We look at our environment and think about everything that can hurt us (physically, mentally, financially...). This in turn makes us worry more and more, bringing a sense of constant anxiety.

Also, with the increase in digital interactions, we share less and less moments with other people. We don't feel the emotions of human contact and confrontation with others in real life. On the internet you can be whoever you want to pretend to be, but you always know deep inside that it's not you. You gain recognition online by not being truly yourself. Not being able to know if you are liked/loved for who you really are is an important factor in making us unhappy. With a more and more digital society it is also harder to find a place in that society, while on the other hand we have easy access to information about all the amazing things we could be doing. Confrontation between our life and the extravagant life of others can deepen the feeling of unhappiness we have.