Why do guys look at butts?

My dad will be cruising in the car with me and he will look outside the window and stare at random female butts... it makes me feel uncomfortable especially since my father has a girlfriend. When I am at work I have caught some men looking at my butt... I don’t even have an ass!! Is it instinctual? Why do men look at random asses?
Why do guys look at butts?
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