Well, there is no generic rule of the kind of "people who go through hardships or get rejected are more likely to be happy where they are", but there is an important factor to note here and this is the comparison factor.

The human mind puts things on perspective based on comparisons. You might have noticed people who were born and raised in poor communities of Africa or Asia, for example, that they seem to be smiling all the time. This is because most people in their community are more or less equally poor. People that are unhappy with their current financial (or other) situation are people who are surrounded by people who are, or are considered to be, better off. A person who makes 90K per year is rich and lucky for the standards of a lower middle class community, but he will consider himself to have failed if he lives between millionaires.

We tend to fall in a form of survivorship bias, a logical error of concentrating on people or things that are successful and compare ourselves to them only, completely forgetting about the ones that have failed due to their lack of visibility. Nassim Nicholas Taleb describes it well in his book Fooled by Randomness (he makes excellent points by the way - I recommend reading his books) in the following example.

You might have seen people who lived better and subconsciously compared yourself to them or wanted to somehow reach them. You might have had enough trouble as a kid to turn you into an independent and restless person as a means to keep coming up with ways to survive on your own. There is definitely more than one single clear influence, it's always a combination, complicated enough to make each person who ever existed on earth, unique.