You are also defined by what you are not.
Describing a thing by what it is not — applied to the self: what you're not is also who you are, and declaring it sharpens the model.
Negative definition is often the sharper instrument. It's easier to be honest about what you'd refuse than to articulate what you want, and refusals are more stable — tastes drift, but "I don't do that" tends to hold for years.
Applied to a self-model, it does something positive systems can't. A hundred stated interests still leave a recommender guessing; a handful of stated anti-interests kill entire branches outright. A veto is more information than a preference, because it's unambiguous.
Which is why it belongs in the model rather than in your head. What you're not is a fact about you — and unlike enthusiasm, it doesn't need re-deciding every time something plausible shows up.