Well, it has to do with the polarity or lack of polarity of the substance. So for instance, like dissolves like = salt will dissolve in water because they are both polar substances. Besides salt being ionic. This is the same reason sugar dissolves in water due to the polarity of both the substances. On the other hand, a nonpolar substance will not dissolve in water(polar). Example, oil (nonpolar) does not dissolve in water (polar) - even if you shake it up. The oil eventually rises to the top (due to lighter density and nonpolarity).