I think; happiness is a choice like other emotions, emotions are fundamentally an illusion and all people have their own subjective conditions that they set for themselves to feel particular emotions.
For example, some people are happy just because they believe; they are handsome/beautiful, they have a cool car, they own an alluring house, etc. but on the other hand, some people can't be happy no matter what.
Opinions like being "handsome/beautiful", having a "cool" car, owning an "alluring" house are subjective therefore conditioning yourself to feel x emotion just because of your own subjective opinion makes the emotion you feel an illusion like, for example, being angry just because someone looked at you. It's nonsense. Emotions are not logical because there is no objectivity of emotions therefore emotions are fundamentally an illusion. So, for these reasons, for example, I become a "happy" person when I read an "interesting" book but other's may find reading a book fundamentally "boring" therefore reading a book may make them feel some negative emotions like we conditioned ourselves to feel so. I think; it makes me really happy but fundamentally it's like I feel "happy" just because I think I should feel so therefore I kinda "really" feel I'm happy like I'm lying to myself by believing I feel happy — I'm having an illusion.
The only difference between the happy person and a not happy person is; former has a "faking it until you make it" logic, latter don't. So, a happy person believes in whatever they want, thus, they create their own reality that is a happy reality to be happy. That's how the brain works: For example, if you believe you are mad then you will be mad.
Happy people choose a reason to be happy, a not happy person doesn't choose a reason to feel happy. You can even think about it as; a not happy person has no idea how to be happy because they can't find a reason to be happy. It's like, they don't even want to choose to be happy.
If you have no reason to be happy, you will be depressed; you will perceive only the negative aspect of reality, you will create your own personal hell, you won't have a positive emotions source to deal with the negativity of life, et cetera.
The reason to be happy has to be found on your own because the reason to feel x emotion is subjective therefore the reason depends on your personality. The reason is not necessarily the same as others' reason(s).
Some people waiting to be happy but because they don't have preset condition(s) to be happy, they can't be happy.
Some people even mistakenly think; what makes others happy will make them happy too. Well, some people think they are happy just because they believe they should be happy because of x reason because they think; "this made x person happy then I should be happy too" but even though emotions are an illusion, believing you feel x emotions even though you do not feel it makes what you feel a delusion of an illusion but because of how the brain works, their delusion of an illusion is perceived as real because they believe they are happy therefore they become pseudo-happy that we can consider as "really" being happy.
What a happy person thinks? They think; they found their own reason to be happy and this x reason should make them happy therefore they know how to be happy. They choose to be happy thus they find their own reason to be happy therefore they are happy.
So, what a not happy person thinks? Well, fundamentally they think; x is not a good reason to be happy. Those people dislike x for whatever reason. It's like they choose to feel negative emotion(s) therefore they dislike, they hate, et cetera. Well, haters gonna hate, right?
I also want to write about free will thus what I wrote so far may make sense to you dear reader if it didn't:
For example, those who don't believe in free will actually able to choose to don't believe in free will because they have free will thanks to their prefrontal cortex. Consciousness allows you to manipulate the information flow in the brain thus you have subjective opinions, you can decide, there is a reason behind your every decision even though you are not necessarily aware of this decision-making process (I call this process as gain and loss analysis), et cetera. Your brain that is under control of your consciousness only can be used if you make the choice to use it. So, you focus if you choose to focus, you can even feel x emotion if you choose to do so because of the ability to manipulate the information flow in the brain. I'm not going to give you neuroscience 101, psychology 101, etc. class here. What I stated so far is enough anyway. If you still believe there is no free will, there is no choice, emotions are fundamentally an automatic process (well, particular emotions become an automatic process if you have a preset condition like conditioning yourself to be angry just because someone looked at you like you must), etc. then suit yourself because you are free to believe in anything you want. People choose their own "rights", believe in their own "rights" thus either they have their own opinion or they are a normie that believes in other's "rights" just because it seems so to them. That's the way it is. Being "right" is a matter of point of view anyway.