The linked answers do attempt to clarify on 'stability' but I wanted an answer with specific connection to resonance.
For example, the ethanoate ion can exist in many forms but, as we know from data, it exists in a form that is a hybrid of all the possible forms. Why is this? I have heard teachers say that this is since that form has 'less energy' or is 'more stable.' I understand - from the linked answer- that these terms are open to interpretation. The criteria my teacher used was that in each of the contributing mesomers, the electrons are localised. But in the hybrid, they are delocalised. Now I understand this results in more 'stability' in some sense of the word.
But is that only criteria ? Should we not also consider the electronegativities of the atoms involved, for example? Basically, why is the hybrid form the form in which the structure exists?