Do you see a pattern forming?

•	The number of π electrons in the system is crucial: when they have 4 or 8 π electrons both cyclooctatetraene and benzene adopt non-planar geometries; when they have 6 or 10 π electrons a conjugated planar geometry is preferred.

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Remember: the planar 1,3,5,7-tetramethylcyclooctatetraene dianion (refcode TMOCKE) still has considerable ring strain. The fact that this structure adopt a planar geometry must mean that there is some other form of stabilization (gained as a results of having 10 π electrons) that outweights the strain of being planar. The extra stability is called aromaticity.