Why are some people happy?

[Been There]

Many people I know always say stuff like "I have no life" or "I am boring" or "I feel like death". Stuff that you say about yourself. My friends in twitter talk like this, but they say they're only kidding.
A: They aren't really kidding. It just sounds too raw and vulnerable once they have said it, so they "retract" it by saying "I'm kidding."

But sometimes I meet people who are genuinely "happy". I think they are called enlightened people.
"Happy" in enlightenment, is not the same thing as "happy" in non-enlightenment.

And I heard Buddhism helps you become enlightened. But Buddhists say that you don't have to be Buddhist to BE enlightened, which is why non Buddhist people are happy.
True, you don't have to be a Buddhist to become enlightened. But remember that Siddhartha Gautama took 6 years ot TOTAL UNDISTRACTED attention to the problem of "Why do we suffer and how can we stop suffering?" And that it was not only the end of those 6 years, when he sat down and meditated for 49 days straight (that is 1,176 hours straight) that he finally saw his inner mechanisms that PRODUCED his unhappiness, and he saw the solution.
I doubt if he actually sat for 49 days straight .. more likely, this is a way to impress on us just how difficult it is to change our perceptions of ourselves and the way we react and interact within ourselves.
And, honey, it IS a slow process requiring incredible commitment and discipline and persistence. It takes 3-8 years of doing the practices even just to budge our psyche off the "starting line", to say nothing of completing the race.

Anyone can achieve enlightenment for themselves, but it doesn't happen overnight.
The state of enlightenment means that you no longer rely on something to make you happy, and you no longer allow anything to make you unhappy.
You achieve this by living in the moment and by relaxing into whatever IS happening. And you are able to live in the moment because you have mastered the art of "mindfulness" through gaining skill at the mental discipline of meditation.
It is not that you no longer feel unhappiness or happiness .. it is just that you have opened up to EVERYTHING going on inside you and no longer try to "fix" it. You just take it for with it is, vibrantly and fully alive to who you are and what is going on inside you, feeling linked with others by virtue of your compassion for our common human state, and living each moment with a richness and acceptance. Buddhists become free from the tyranny of Unhappiness, and also from the tryanny of Happiness (likely you won't grasp that until you have practiced Buddhism for some years).
The more you think you need this or that, or need to avoid this or that .. the more unhappy you will be with life.
And the more you focus on yourself, instead of others, the more unhappy you will be (actually, modern psychology says the same thing: the more self-centered you are, the more neurotic and unhappy you will be).

How to explain it?
Are you at the point where you can get a shot in the arm and it's no big deal? Where you hardly even feel it, and your emotions are not disturbed by even a ripple of upsetness? That means you have learned to relax into the experience.
Or do you still tense up with dread and fear, and find the whole experience both painful and terrifying? That means you are resisting.
And resistance always increases pain: both physical AND emotional pain.
Buddhists .. on the other hand do not try to fix life (they don't run from the "needle") .. they just let it happen and relax into it, residing in that focused awareness called "mindfulness".

Yes, you can learn how to do this without doing the practices .. if you are willing to put the same kind of focus and energy into it that Siddhartha did.
Unfortunately, even the monks in temples don't put THAT kind of focus and energy into it.
The odds that you can do it without the specific techniques and practices of Buddhism, without the guidance of someone who is further down the road than you are .. the odds are very low you can stumble upon the way to become happy on your own.

So I wondered, well, what's the trick? I wanna be happy and not mean or miserable. I keep hearing smile a lot. But that sounds stupid,cause, peoples will think you have smoked weed, if you're constantly smiling. Smiling a lot does not make you happy. It's not that simple.

You are right .. "smiling a lot" is stupid. It does not work.

"Mia" and her answer are on the right track.
The only thing is that we are talking about changing our unconscious habits. Habits are difficult to change and require constant and EFFECTIVE behavior change techniques. It takes years to change ourselves. it's not just some "mind trick" on how you see things .. but that IS half of the equation.
The other half is to discipline and train the mind (meditation), so that you can see - without distraction - how YOU work inside yourself. That produces inner insight, and that is the source of our change and the development of our wisdom.
Even practicing Buddhism.