Definition of success is changeable according situation.
Put the word "the" between "according" and "situation".
However successful is not related to the circumstance of wealthy, its subject is about the happiness.
Put a comma after "However".
If we observe the successful people around us, we can easily recognize that this kind of people had a chance at past and they used this opportunity to have a good life.
This is a little messy, it should be written like this: "If we observe the successful people around us, we can easily recognize that these kinds of people had a chance in the past and they used that opportinity to have a good life."
Every year 200 thousand people enter this exam and only 2 thousand people can deserve to attend a university.
If you are going to write out numbers, write out the whole number, not just part of it. So, that should say, "two hundred thousand" and "two thousand".
Other than that, you have written an interesting essay. I would have liked to have seen more examples though.