Oxygen already has the tendency to add two electrons, filling the capacity of its outermost electron orbitals. Its high effective nuclear charge enhances this preexisting tendency and is manifested by a relatively thin electron cloud around the oxygen nucleus. In other words, the attractive forces of an oxygen nucleus are not as well shielded by its orbiting electrons as other similar elements with bigger electron clouds. The high electron affinity of oxygen is why the chemical reaction called oxidation, or electron loss, is named after it. Oxygen readily captures electrons from other substances.
Combustion is a type of oxidation reaction.