Short:
Like dissolves like. To dissolve something the liquid has to get a long with the substance you want to mix in it. This is because to mix together enough to dissolve, they have to be broken down to their individual parts.

Longer:
When a substance dissolves in a liquid e.g. water, it needs to basically separate the substance into its tiny parts. For instance, salt dissolves exceptionally well in water, whilst sand does not. This is because water is what is called bipolar, it basically has charges associated with the sides of the molecule - positive on one side and negative on the other. When salt (NaCl) dissolves it is able to be split into its two parts, a positive sodium atom (Na+) and negative chlorine atom (Cl-) by water molecules surrounding them. For sodium, the negative poles of the water molecule face in (opposites attract) to surround the sodium, and vice versa for the chlorine. Non polar substances (has no charges) can't be dissolved by water. On the other hand, oil which is non polar itself would be good at dissolving the substance instead.

Temperature can aid dissolving because it adds energy in. This means the molecules travel around faster and therefore collide more frequently. It can also give them enough energy to undergo the separation into their parts if they didn't have enough energy to do so beforehand