I would say it is highly subjective, but not purely subjective. People are a lot more likely to like the smell of fresh fruit than the rotten eggs smell of hydrogen sulphide. And it isn't universal, we humans are likely to be repelled by the smell of rotten meat. There's good reason for that, the smell warns us danger. But if we were flies, that smell of rotting meat attracts rather than repells. There are smells in the middle, which aren't terribly good or bad on their own, but association with good or bad things can cause us to have positive or negative emotional reactions.
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