Hi,
Here are my thoughts. I'm neither an English teacher nor a native English speaker, but an English learner as you are, so think twice before following any of my suggestions. I hope, native speakers will provide their opinion on your essay (and perhaps my comments) too.
* I didn't understand why the expression
"believe it or not" is used here. It usually precedes a statement that many people will find arguable, but I believe that most people will agree that the happiness is one of the central concerns in the life.
*
"concerns of lives" - why the noun "life" is plural here?
*
"which nationality you belong" - I'd add "to" to the end of this expression.
*
"it is happiness you really care about" - Here you cannot omit the relative pronoun "that" after "happiness", because it ("that") is a subject of a relative clause (
http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv336.shtml).
*
"everyone want" -> "everyone wants".
*
"It may due to the fact" -> "It may be so due to the fact"?
*
"various from person to person" -> "varying from person to person"?
*
"for his or her own happiness" -> "for one's own happiness"?
*
"could not be fully understood by others" - "others" usually means "the rest", "different from ones mentioned before". In your case I didn't understand who are these others.
*
"the 2 different happiness" - I'd better use "two" insead of "2". It's always better to use the literal equivalent instead of numeric number representation.
*
"those ambitious people" - Again, I don't understand who are you refering to by using "those".
*
"often it is impossible" -> "it is often impossible"?
*
"to be the billionaire" -> "to be a billionaire".
*
"Besides, you need to focus on it and try to make it come true." - what "it" is referring to here? I believe, it originally referred to goals achieving. Then the "billionaire"-sentence was inserted, so now "it" probably refers to the love of money.
*
"study in Oxford University" -> "study at Oxford University".
*
"studying hard and let yourself be" - I suppose, "let" must be used here in the form of gerund.
*
"Thus, you have got the fundamental item to step to happiness." - I don't understand how this sentence is connected with previous ones ("thus" suggests that the connection exists, but I failed to recognize it).
*
"There may be other factors in achieving happiness, but I believe these two are the basic factors to grasp your happiness. So, let us try our best to get our happiness." - I'm not quite sure that the word "factor" can be used here with the implied meaning. And another point - "happiness" occurs too many times in these two sentences. I'd restructure them.
P.S. I recommend you revising each your text after it's been refactored and trying to reconstruct all the story from what is written as if you haven't written it. Place yourself in the readers position and I believe you'll find lots of places in your text that at least need clarification.
I hope, it will be helpful.