Dimerisation is a process of association (addition reaction) of two similar molecules in order to gain stability which was absent in the monomer form.
NO2 being unstable dimerises to N2O4 to exist as a stable molecule.
NO2 beforehand, was an odd electron molecule (7+16=23) due to which it tries to unite with another of its kind to become stable.
That is why it dimerises.
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