Very few elements are composed of individual atoms that arent part of a molecule or crystal in their native form under ambient conditions. Individual Oxygen atoms dont have a full shell of electrons. They are extremely reactive for this reason and will pair up to form the considerably more stable O2 molecule. As for why it is more stable, when atomic orbitals interact with one another to form molecular orbitals, the number of molecular orbitals that form is equal to the number of atomic orbitals used to form them. Generally there are two classes of molecular orbitals: bonding and antibonding. Bonding orbitals contribute constructively to the formation of bonds. Antibonding orbitals contribute destructively to the formation of bonds. I.e they weaken bonds. There are a few analogies that can be used to sort of understand what is going on. Constructive interactions are like pushing someone in a swing at just the right time to add to their back and forth motion OR swinging a jump rope at the right time to maintain the wave. Destructive interactions are essentially the opposite of this. An analogy would be pushing someone in a swing at just the right time to slow their motion until they stop or swinging a jump rope at the right time needed to cause its motion to slow until it stops oscillating. If you have experience with tuning forks, youll also notice that if you couple two vibrating tuning forks together you get two separate sets of possible frequencies. One lower and one higher. This has the effect of stabilizing the system. Something similar happens when molecular orbitals form from the interactions of atomic orbitals from individual atoms forming molecules. You can also visualize this extra stability in the molecular orbital diagram for O2 vs the individual atoms: https://chem.libretexts.org/Courses/Pacific_Union_College/Quantum_Chemistry/09%3A_Chemical_Bonding_in_Diatomic_Molecules/9.10%3A_Molecular_Orbital_Theory_Predicts_that_Molecular_Oxygen_is_Paramagnetic