Why do phospholipids form a bilayer in the cell membrane?

When the phospholipids form a bilayer, they do so in such a way that the inside becomes hydrophobic and the outside becomes hydrophilic. It's essentially two rows of side by side phospholipids, except the rows are "tail-to-tail". So the nonpolar (hydrophobic) tails create a barrier in between the polar (hydrophilic) heads on either side. This keeps many different unwanted or harmful substances out of the cell.

I'm not sure if that helped or not, but I hope it did:)