What chemical property makes lipids hydrophobic?

The chemical bonds between the carbons in the hydrocarbon chains (and between the carbons and the hydrogen atoms) are non polar. Water is a very polar molecule and as such doesn't "recognize" non polar atoms. So because lipids are non polar they don't interact with the water, and remain clumped together, apart from the water molecules. The exception is phospholipds which have a polar end on the molecule and therefore have both hydrophobic and hydrophilic parts.