First off, it is debatable whether Earth is actually the only planet humans can colonize, but in any case, a large amount of effort would have to go into making other planets habitable. Earth's conditions are ideal for human life. The atmosphere contains exactly the right amounts of oxygen, water vapor and other gases for us to breathe, and is thick enough to protect life from harmful radiation and small objects from space, but is thin enough that we are not crushed under its pressure. The temperature is just right for liquid water, which is necessary for life, and there is ample food to sustain us. Also, the Earth's tectonic and volcanic activity is not so violent that civilization is constantly recovering from catastrophe. Humans could likely colonize the Moon, Mars, and other planets and satellites with solid rocky surfaces. At first, they would have to live in space-station-like interiors, but there is a process called terraforming, or rendering a planet habitable, that is currently being researched.