Why do some metals melt?
In solids, the atoms/molecules are held in a fixed arrangement by bonds, which are governed by electrostatic forces. Liquids occur when the atoms/molecules have more energy so that they overcome these electrostatic forces to an extent and move about a bit.
Different materials form bonds in different ways. If you are talking about elements and compounds, then they will all be a liquid at some point given the right amount of energy (or a high enough temperature). However, lots of metals we know are alloys. Most alloys do not have a single melting point but instead have a melting ‘range’ in which the alloy’s a mixture of the solid and liquid states.