I think that maybe the customer was making a (LAME) jab at the cashier for saying she HAD to pay, she had no choice.
Anyway, maybe she was referring to how chocolate causes similar chemical reactions as sex, "love, general happiness (dopamine, serotonin. From Wiki.: "Phenethylamine, an endogenous alkaloid sometimes described as a 'love chemical... it is quickly metabolized by monoamine oxidase-B and does not reach the brain in significant amounts"). Honestly though, I really don't think it effects MUCH. It tastes good, many people get happy feelings when eating something tasty. I think it's a simple as that.
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"Read a f#@^ing book" - Nucky Thompson, "Boardwalk Empire"
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"We have neither of us anything to tell; you, because you do not communicate, and I, because I conceal nothing." - Marianne, "Sense and Sensibility&