I have mixed feelings about the advice to "be realistic." It often doesn't mean what people think it means. It doesn't refer to Reality (capital R), but the present-day reality we find ourselves in. It does not take into account the abundant resources on earth (and, perhaps, in the universe) and the infinite creativity of human beings to innovate and change.
"Be realistic", in common usage, is to be unrealistic. It overlooks how far we've come and what's possible for us in the future. Our present-day world would either be understood as magic or a miracle for people who lived a hundred years ago. How accurately can we predict what our future will be 50 years from now? And how "realistic" would our predictions be?
Being realistic is also an excuse people use to absolve themselves of responsibility to bring about change, and they admonish others to do the same so their shortcomings don't become too apparent and they don't feel guilty for not playing an active part in addressing the problems we're facing.
If you truly want to be realistic, then you should also be optimistic about what is possible for us and to be enthusiastic in playing your own part in making the world a better place.