Tears are composed of salt and water because the human body has high concentrations of both salt and water. Salt is found in virtually every cell in the human body. In total, there is around 1 cup of salt in the average adult human body.
Salt keeps cells hydrated by pulling water into cells when necessary. When people cry, plasma fluid is pulled from the cells in the lacrimal gland and is secreted through the tear ducts. This fluid is full of salt, electrolytes and metabolic waste products of cellular processes. Tears are a way the body secretes these products to get rid of them.