For us, Earth is the only planet that will support our life forms. That is because we developed here, and so we were formed to need a certain amount of warmth (but not too hot!), and a certain combination of molecules in our air. Now, if we went to live on another planet, it would be too hot or too cold for us, and its atmosphere would not give us what we need to breathe. If we had developed on that planet instead, then we would have adapted to it. Of course, then we would not be ourselves!
Answer provided by Dr. Universe