There are certain fields that are relatively concrete. Math and Science tend to have one correct answer (excluding theoretical nonsense). If 1+a=3, a=2 and there's not many ways around that. If chemical x reacts to chemical y in a certain way, there are only a certain amount of ways to explain it. If these concepts were not concrete and unchangeable, and instead open to interpretation, the world would be chaos and it would be practically impossible to move forward.
There are certain fields that are not concrete. Sociology, English, and Philosophy are up for a fair amount of speculating, theory, and malleable subject matter. You can argue that Romeo and Juliet is a story about love, or you could say that it is a criticism on the wastefulness of youth. Neither is correct, and neither is incorrect. Unless you have a direct quotation from the author stating what the piece is and isn't about, the subject is eternally up for debate and discussion.
Critical analysis of drama and literature have a methodology to them, yes, but there isn't a formula that you use to reach a singular, indisputable answer. As long as you're knowledgeable in the various aspects of the piece, you are as adept as reaching your own conclusion as anyone else.
To simply say that a person's individual interpretation of a creative work is incorrect because others have not reached the same conclusion is ridiculous.