Nicotine and caffeine

[Pillowpants]

To all the plants in the Nightshade family that contain nicotine it serves as a pesticide, a deterrent to herbivores and insects that would like to munch on it. Yet nicotine also has addictive properties. Why would some plants, on the basis of survival, evolve nicotine as a pesticide to aid in survival and yet have these critters addicted to the chemical pesticide? The same goes for caffeine too. Please tell me if any of my information is incorrect, it just seems contradictory.

[JackBean]

I don't think it's addictive for the insect, is it?

[Rap]

The parasitic insects are repelled, animals and humans are addicted. Both effects increase the fitness of the plant. Insects cannot destroy the plant, but animals eating the seeds propagate the plant. Animals do not destroy the seeds, they pass through the animal and are planted (along with some good fertilizer) somewhere else. Human addiction propagates the plant too, it causes the plant to be treated as an agricultural crop.