In chemistry, we learned how molecules go around looking for atoms. In a negatively-charged state, they will look for atoms and molecules with a free electron to donate and then pair up. They will seek this until they can fulfill themselves from their unfinished state.
People aren't so different. They bounce around the solution of life looking for a complementary mate. "I need __________, is that you?"
Two lonely oxygen atoms pair up with hydrogen atoms. Double date!