Metals lose electrons to form ions, a process that typically occurs between metals and non-metals. Because metals have a very low electronegativity, they lose electrons easily to high-electronegativity non-metals.
Metals generally have very few electrons in their outer electron shell. Because an atom always seeks to balance out the number of electrons in each of its shells, the atoms in metals are eager to lose the lone electron in their outer shell. Whenever two outer shells come into contact, the one that needs an electron to complete its electron set easily takes the lone electron from the other atom. ChemGuide explains this phenomenon as electronegativity.
Georgia State University clarifies the nature of electronegativity, stating an atom's ultimate goal is to form a polar bond. Metals are fairly unstable elements, known for their ability to form bonds rapidly. Princeton notes an ionic bond is formed when a metal and a non-metal exchange one or more electrons. This exchange occurs because metals and non-metals are polar opposites, each reacting to the opposite charge of the other. Although it is possible for a non-metal to lose an electron to a metal, the opposite is the ideal exchange and far more likely to occur under normal circumstances.