The logic of how an individual purchases the utility maximizing quantity of a single good is incredibly simple. Remember that marginal willingness to pay is the maximum amount she is willing to pay for one more unit, and that marginal willingness to pay is diminishing as consumption increases. This means that she is willing to pay less for each additional unit than she was for the prior ones. Marginal willingness to pay is what one more unit is worth to our consumer; so as long as it's worth it, she'll buy, and if it isn't she won't.
Simply put, our consumer will buy every unit for which marginal willingness to pay is above the market price, and will buy none for which the marginal willingness to pay is below market price.
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