Why can cross price elasticity of demand be positive or negative, unlike the price elasticity of demand with respect to the item’s own price?

Cross price elasticity of demand is the responsiveness of the demand for one good to a change in the price of a related good. Goods are related in two ways: complements and substitutes. For complements, the demand for one good increases in response to a decrease in the price or another good, so that the cross price elasticity of demand for complementary goods is negative. For substitutes, the demand for one good decreases in response to a decrease in the price of another good, so that the cross price elasticity of demand for substitutable goods is positive.