We tend to assign one of three labels to a situation, a person or an outcome of a deed: good, bad or no label. Krishna refers to this third state and says (2.57) that a wise person is one who isn't filled with joy when coming across good nor does he hate bad and is always without attachment. This implies that the sthithpragna (one with coherent intellect) drops labelling(2.50) and takes facts as facts without any extrapolation, which is the birthplace for polarities of pain and pleasure.
This verse is tough as it runs contrary to our tendency of instantaneously labelling facts as good or bad even in moral and social contexts. When one encounters a situation or person labelled as bad, dislike, aversion and hatred follows automatically. On the other hand, the sthithpragna doesn't label it and hence the question of hating doesn't rise for them. Thus similarly when coming across good, the sthithpragna doesn't get overjoyed.
For example, all of us go through the natural process of ageing with time where beauty, charm and energy are lost. These are mere natural facts, but if we label them as unpleasant or bad, then this labelling would bring dukh(sorrow). Same is the case with injury or illness where labelling of these as evil brings sorrow. Certainly, it's neither denial nor extrapolation.
The sthithpragna handles situations like a surgeon who is expected to perform surgery based on pure facts brought out during the investigation. It's like a super-conductor that does its best to let all electricity pass freely.
We tend to either cling or averse to situations, people or deeds. It's easy to understand clinging as attachment, but aversion is also a kind of attachment, however to hatred. When Krishna says that sthithpragna is without attachment, he means that they drop both clinging and aversion.
Source - Daily World