In the above mechanism's first step, what motivates the use of the carbanion resonance structure of the enolate ion if the negative charge is better localized on the more electronegative oxygen?
Edit: I might not have been clear in what motivates my confusion. Maybe I should rephrase what I'm asking. Basically, why would the carbon on the enolate attack the carbonyl carbon as opposed to the oxygen on the enolate attacking the carbonyl carbon? I assume there's a mixture of both products, but why is the aldol the major product?