Yeah I think about it all time... I am chemical engineer and since I start to study I decided to make something to help many companies like petroleum companies using products environmental friendly. 

I develop a product from acid lactic bacteria for enhanced oil recovery. 

Thats what I do for saving the planet. 

:D

Making the world better will likely make you personally feel better. There are a lot of studies that show this effect. So I think it's possible :)

I find that when people talk generally about "changing the world" that they are thinking about doing something grand and majestic but we all have the capacity to "change the world" in our own small way which may not ever be recognized (being well known for something doesn't necessarily make it  a better accomplishment).

For example, suppose you are a case worker and help at-risk youth complete school and go to college.  Perhaps you live in relative obscurity but someone you help is saved from making bad choices and goes on to do something big and exciting.  Did you change the world?  I would argue yes, but maybe not in the way you had first imagined.  

Like @Sandra says, if you are doing things in your life that are meaningful and fuel your passions, who is to say you aren't making the world a better place simply thru your influence on those around you?  The opposite, a world where everyone is completely self-centered and has no passion beyond advancing their own popularity doesn't seem very great either.

I believe making personal life and family life better and then working for the world both are not possible in one life. Either we get busy in making ourselves happy or we sacrifice personal happiness for the greater good like starting an NGO or join military etc. 

Making a chage in the world has always been very tough and people who step in this path should be super strong , otherwise there won`t be any results.

People who want to change the world should know that they won`t be admired in the proccess of changing the world and they must get through a very hard time but as long as you have a strong mind and personality , it`s possible to make it.

And i have always wanted to do sth for getting the world a better place for living (most f the times i failed:)and if i can`t do this i help thoses people who are making some effective changes.

:)