Humans have red blood because it contains iron. Another way of explaining why humans have red blood is that the color is due to hemoglobin, which is the protein associated with iron that makes blood red.
Hemoglobin consists of four chains, two of which are alpha chains, and two of which are beta chains. Each of these chains contains an iron atom. Oxygen combines with the iron atoms and is carried through the blood.
Not all species have the same color of blood. Crabs, snails and other crustaceans have blue blood because their blood contains copper. Sea shrimp have green blood because it contains vanadium.