Briefly, amphipathic molecules spontaneously assemble into micelles because these more organized structures, paradoxically, represent lower free-energy states than are present when individual molecules are in solution, completely surrounded by ions and water molecules. The nonpolar portions of the phospholipids are excluded from the ionic saline solution (and dipolar solvent), due to what Charles Tanford (1980) has called the hydrophobic effect. Literally, energy (as heat or in some other form) is required to force a micelle to break apart into individual phospholipids. Additionally, the hydrocarbon chains themselves, forced into close promity by hydrophobic exclusion, can become more structured and stably associated through inducible dipole-dipole (Van de Waal) interactions along their entire length. Keep this last property in mind when later we consider the effects of "kinks" in lipid structure on membrane properties.
These properties also affect the interaction of amphipathic regions of larger molecules such as proteins and the association of membrane proteins and membrane lipids.