C6H5CO2H dimerization in Benzene Why does $\ce{C6H5COOH}$ in benzene form dimers? Also why only in benzene, why not in other solvents such as water?

In general, what are the properties of compounds which form dimers or polymers in benzene?
In polar solvents such as water, the water molecules will solvate the carboxylic acid. Thus, hydrogen bonds will form between the 2 molecules and the carboxylic acid will dissolve.

The water is in large excess as it is the solvent so it will solvate most of the carboxylic acid molecules.

In a non-polar solvent such as benzene, no such hydrogen bonds can take place with the carboxylic acid and benzene molecules. As a result, the carboxylic acids simply interact with themselves and form dimers.