Here are four major reasons that human beings are able to survive on Earth as opposed to any other planet in our Solar System.
1) Water is the common denominator for all life on the planet, particularly mammals like humans. Water makes up the largest percentage of the mass of a human body, and it makes up the majority of blood plasma and cellular cytoplasm. In essence, water is the chemical solvent of the human body that all of its biochemistry takes place in. Fortunately for us, the Earth is also mostly comprised of water, although only a very small percentage of it exists as useable, potable water (most is saltwater or frozen arctic ice).
2) If water is the liquid solvent of life, then oxygen is the gas of life. Oxygen is probably the most common oxidant on the planet, and it allows for the process of cellular respiration whereby humans convert food molecules into a useable form of energy (ATP). Fortunately for us, the Earth's atmosphere is about 20% oxygen.
3) A moderate temperature is also critical to human life on Earth. The distance of the planet away from the Sun coupled with the atmosphere that keeps heat near the surface of the planet allows for a relatively mild, constant climate without too many radical shifts toward either extreme over most of the planet. This is not only important for the human body to tolerate, it also allows for water to exist mostly in the liquid form as well.
4) The fourth factor is food supply. Food for humans exists in numerous forms on Earth, from plants that produce their own food via photosynthesis to other animals and fungal life A huge variety of other life forms on this planet are readily edible for humans to comfortably exist.
The four major reasons that enable human to survive on earth are :-
1. Water - Its importance can be seen in different ways. Water makes up the largest percentage of the mass of a human body . 3/4th of earth is also animals. Water plants and animals were the first higher organisms to emerge
2. Air(Oxygen, Nitrogen and Carbon dioxide):- oxygen helps in respiration ,nitrogen reduces the burning capacity of oxygen and carbon dioxide for plants on which all living beings depend for food.
3.Optimum temperature around 391 K.
4.Sunlight as primary energy source, land availability for making houses and growing food , food supply from plants and animals.(anyone could be taken as fourth major reason).
In addition to the answers above, Earth has a rather unique advantage when it comes to it's atmosphere. Unlike mercury (which has very little atmosphere at all) or Venus ( which has almost 90xs the atmosphere of Earth) Earth has an atmosphere that perfectly supports life. It has just the right amount of hydrogen and sulfur to deflect dangerous UV radiation but not so much so that it suffocates it's inhabitants.