
Anton I. answered 05/24/19
Biomedical Sciences Masters Candidate. 3+ Years Tutoring Experience
Extracellular fluid is water, so is cytosol inside the cell. The hydrophobic tails of the phospholipids are much longer than hydrophillic phosphate heads. So tails "want" to be as far away from water as possible and "heads" want to interact with water as much as possible. The double-layer of the membrane optimizes this arrangement and makes it more thermodynamically stable than any other combination. Think of it as a magnet that wants to align its +/- poles with the direction of the magnetic field. There are also electrostatic forces between the tails.
Also, things move very fast on cellular level, so individual phospholipids sometimes flip in opposite direction, but return to their low energy configuration.