Actually, at all times dilated pupils serve a practical purpose- as you say ^ they enlarge so you can see better in the dark, but when you look at someone/something that you find attractive, they also dilate, simply to send more light in the visual centres of your brain, so you can behiold the object of you attraction better. Thats the point of pupils, to regulate light flow into the eyes . . . i actually find I get paranoid having big pupils because their must be some danger in having like twice the mount of light enetering them for an extended period. Anyone whose had acid or the like will be familiar with those strange colored blobs at the edges of vision, which are actually caused, not by the hallucinogenic substance, but by prolonged pupillary dilation. Also, when I'm on drugs, I prefer it be less public thing then striding about with massive black dots for eyes.