why some people didn't contented for what they have now?

maybe i am those who are not and will never be contented. i mean, i am a happy person and i am happy for who i am and what i have become now. however, i still keep on dreaming for something better and that makes me not really feel satisfaction 100%. it's nice to say that i am perfectly contented since i still do dream of big hopes for the future. but i can say that i am happy and thankful at this time of my life.