Here's why:

* Polarity: Water is a highly polar solvent due to its hydrogen bonding. Triethylamine is a non-polar molecule, as the three ethyl groups (CH3CH2-) attached to the nitrogen atom distribute the electron density evenly. This difference in polarity leads to poor interaction between the two molecules.

* Hydrogen Bonding: Water molecules can form strong hydrogen bonds with each other. Triethylamine, lacking a hydrogen atom attached to an electronegative atom, cannot participate in hydrogen bonding with water.

* Hydrophobic Effect: The non-polar ethyl groups in triethylamine interact favorably with each other, creating a hydrophobic "shell" that resists interaction with water.

In summary, the lack of polarity, inability to form hydrogen bonds, and hydrophobic effect all contribute to the poor solubility of triethylamine in water.