Why does AlCl3 dimerise to Al2Cl6 at low temperatures?
Perhaps, we can employ the Gibbs free energy equation for spontaneous reactions...

$$\Delta G=\Delta H-T\cdot\Delta S$$

Since the dimerisation process involves the formation of bonds, $\Delta H$ should be negative, while $\Delta S$ will be negative too, since two $\ce{AlCl3}$ molecules become one $\ce{Al2Cl6}$. As such, when temperature is low, the magnitude of $T\cdot\Delta S$ will be smaller, and $\Delta G$ will be negative or more negative, thus favouring the dimerisation process.