Chemistry is the science of materials, their properties, and how they react with each other. All materials are made of atoms. Why do some atoms combine with each other while other atoms do not? Because atoms just want to be happy!
Of course, atoms don’t really have feelings. But they do have observable behaviors. And atoms tend to be more stable, or “happy,” when they have certain numbers of electrons whizzing around their cores. Where do they find extra electrons if they need more to be happy? From other atoms that have too many electrons, of course! When atoms share or exchange electrons to be happy, they stick together. Happy atoms form molecules.