[NSFW] If chemical reaction is what causes arousal why is it not the same for everyone?

If sexual arousal is supposedly triggered by chemical reaction such as pheromones, dopamine, etc. why is it not the same for everyone? One man sees a woman in a short skirt with shapely legs he becomes aroused, while another man could care less, but if that same woman passed wearing a particular perfume then he is aroused; not by how she look but how she smells. Another man is aroused because he sees a woman with large breasts wearing a snug sweater, yet another man is on a nude beach with throngs of naked women and does not become aroused. A woman sees a guy shirtless and get aroused, her friend doesn’t notice and it has no affect on her, but she is aroused because of what tattoos he has and/or where he has them, has hair on his chest, or he has a voice in a certain tonal range, etc. If it is purely a chemical reaction that is the catalyst or aphrodisiac, should it not affect every human the same way as if it were arsenic, or Novocain?