Why do metals tend to lose electrons to form postivtive ions?

Metals tend to form positive ions because their electron structure causes them to do so. We need to talk briefly about what this means, so put on your thinking cap and grab a seat.

We know that electrons form up around atoms in shells (energy levels). And only a certain number of electrons can occupy a given shell. Further, atoms tend to want to have outer electron shells that are filled to capacity, just like the inert or noble gases. To that end, atoms will loan out or borrow electrons, and this is the basis for chemical bonding.

Metals are atoms that have incomplete outer electron (valence) shells. But they are also atoms that have a lot fewer electrons in their outer shell than it takes to actually fill that shell. (This is in contrast to the nonmetals, which have outer electron shells that are nearly full.) Because metals have "so few" electrons in their valence shells compared to what it would take to fill them, it is "easier" for these atoms to loan out electrons rather than borrow electrons to reach a point where they are emulating an inert element.

Because metals loan out electrons to reach a point where they are "like" inert gases, they will end up as positive ions. And this is why metals form positive ions.