Like @Estafantom wrote: That's for those who have erotic or sexual, or even just platonic (deeper rooted, more family than "pet") feelings and as such emotions for specific animals, like asking why a specific porn lets a random citizen gain arousing state, even without doing anything manually.
Or to formulate it differently: All of the top-models world wide could dance naked before me and invite me to sexual activities - and I would just plain ol' decline, telling them to enjoy a tea with me or leave and don't annoy with sexual activities in front of me. ?
If I see specific animals, that's different. It's then comparable to what I guess people feel if they meet a cute girl or man. ?
As such the "why" is not something able to get explained. It's personal interest, personal attraction, love.
The ""what" do you find (sexually or platonic) attractive" would be able to get more specific responses.