So, you have a nice apartment and a decent job. You never have to experience hunger, cold or any other life threatening scenarios. You have it better than the vast majority of mankind. Why are you not among the happiest people on the planet?
From time to time, we leave our rational, reasonable and organized left brain environment which is responsible to solve all the daily issues. Sometimes we harshly get kicked out of it - by the death of a beloved one, a serious disease or something similar.
We then go into the think tank about our desires and what is truly important. Sometimes combined with the intention to fundamentally change something in our life or maybe even to create some greater life plan. This is a creative and intuitive task - the right brain environment. Some of us want to be healthier, some want to be wealthier. Some want better relationships, some want more esteem. And some are looking for their life purpose.
I find Maslows Pyramid (of human desires) a cool explanation to all these questions (source: https://www.simplypsychology.org/maslow.html):
Humans have different layers of desires. The bottom layers need to be satisfied first - the further down, the more urgent they are.
If your life is threatened by war, starvation or a harsh environment you certainly won't think about having a better apartment, more money, a higher reputation amongst other people or whatever it is you are missing right now - in this case you are on the most bottom layer, where all the other layers are meaningless (if you want to simulate this, do a cold shower or even an ice bath - believe me, while you are plunged into the cold, you won't be thinking about any of your 'usual' problems. Thats great relaxation for the mind).
The person in the introduction example has mostly satisfied the basic needs (layer 1 + 2). He (or she) has unmet psychological desires. That could be a missing intimate relationship, not enough friends overall, missing appreciation or even prestige. The catch is: layers/ desires that you already have satisfied will contribute to your overall happiness for only a short time - especially if you did not have to fight hard for it (like surviving in a war). Satisfied layers don't provide lasting happiness.
This is some biological mechanism to force the individual to ensure its survival at all times (by pushing itself further and further). Thats probably one of the most important reasons why man has become the dominant species on the planet, but at the cost of the happiness of the individual - the successful species are not always those with the happiest individuals (maybe never, but thats another topic).
Anyways, with the help of Maslow's Pyramid you might get a good idea what your current desires are. Find them out and then think about possibilities to satisfy them. And, even more importantly, keep in mind the biological human drive to never stay completely satisfied after meeting your desires - to develop a more focused and relaxed-confident mental attitude.