Lipids and water do not mix because lipids are nonpolar molecules without a charge, so they do not attach themselves or bond to charged (polar) water molecules. Only charged polar molecules will attach to other charged molecules.
A molecule starts to accumulate a charge when the atoms and electrons inside of it are distributed unevenly. A water molecule has a charge because the hydrogen atoms and electrons weigh down different sides of the molecule unevenly, attracting the molecule to other similarly uneven, weighted molecules. When water molecules come in close contact, they attract each other and will bond, forming a large mass. As a result, water molecules draw near each other and force out other nonpolar molecules, creating the effect of separation that is seen when oil and water come in close contact. This is the reason why oil does not blend with water when the two are combined.
Even though oil and water are both liquids, they do not mix because the molecules do not bond together. Molecules that are charged and will bond with water are labeled hydrophilic, and molecules that are not charged and do not bond with water are hydrophobic. Oil, a known lipid, is considered hydrophobic.