If you are an introvert, you can fantasize yourself as an irresistible extrovert. If you feel inferior, you can fantasize superiority. If you are uncoordinated or clumsy, you can imagine yourself as a graceful athlete. Sins of the mind don't need to wait for an opportunity. The mind can engage in sin anytime, anywhere, under any circumstances.
People keep returning to the same wicked musings to glean further illicit pleasure.
And the habit of engaging in such thoughts takes root quickly and easily. I have worked with many couples whose marriages have been threatened by adultery. Sin incubates in the mind. The adulterer's heart is always shaped and prepared by lustful thoughts before the actual sexual behavior. Similarly, murder is often the product of anger and hatred.