[MazlovesLee]

Why does Benzene not lose its ring in an electrophilic substitution reaction?

I know it's to do with stability but I'm not sure how to answer it  

[FeLiXe]

I think the answer you are supposed to give is that in a substitution one group is exchanged for another one on the benzene ring. Then you still have the three pi-bonds for delocalisation.

If you did an addition then one pi-bond would be lost and you would not have delocalisation stability any more.

[movies]

For these questions I think it really helps to look at the mechanism.  If you do, you will see that in one of the intermediates the aromaticity of the benzene ring is disrupted.  However, this intermediate can then be deprotonated to restore the aromaticity.  So while the net transformation looks like just a substitution, mechanistically it's more like an addition followed by elimination.

Two ways of talking about the same process!