AlCl3 dimerises to Al2Cl6 whereas BH3 does not. why?

Answer 1

BH3 dimerizes to form B2H6.
I am not aware of AlCl3 dimerizing. Check to see if you wrote this backwards.
Things to consider:
The diborane structure contains two B-H-B two-electron, three-center bonds. This reduces electron density on hydrogens.
If Al2Cl6 has a similar structure, then Cl will have reduced electron density. As an electronegative element, Cl does not like to give up electron density.
Thus I guess this question is written backwards.

Answer 2

It is absolutely certain that BH3 does dimerize to B2H6. This is the famous three-electron bond. Duh!