As the rule of thumb goes, "like dissolves like." Water is a polar solvent, ethanol is polar, whereas ethane is nonpolar. Ethanol molecules contain hydrogen atoms directly bonded to oxygen atoms and demonstrate a partial-positive charge, meaning that the hydrogen atoms are capable of forming hydrogen bondings- the strongest type of intermolecular interaction- without lone pairs on other atoms. In the case of dissolving ethanol in water, molecules of ethanol form hydrogen bonds with lone pairs on oxygen atoms in water molecules to release significant amounts of energy. On the other hand, ethene has no such structure that allow its molecules to establish hydrogen bonds and release little energy when hydrated with water molecules.
It takes energy to take molecules of the substance dissolved appart. The energy of hydration offsets this energy input in the case of ethanol but not ethene. The dissolving of ethanol is thus energetically favored at large amounts, whereas ethane-water mixtures tend to be most stable when in separation.
