For better their lives as well as the locals' life I think it is most of them. For feeling rewarded or fulfilled for doing that, I think depends on both the person and the country, but probably quite rare.

I tried two countries so far (>5 year in both) and there were pluses and minuses in each. Which are different to the pluses and minuses in my home country.

I think that one will (generally) evolve and adopt some habits of the country you immigrated too, while giving up some habits you had before. The result? You might be a more complete person (because you become aware of the habits, and can choose to some extent) but on the other hand you will not belong anywhere any-more (you will not adopt some stupid habits of the new country, but you did gave up some stupid habits that you had).