Sigmund Freud, (pictured to the right) suggested that people find pleasure from watching other people who are not aware of their observance. This concept is commonly known as Voyeurism.
"Voyeurism is a perversion in which a person receives sexual gratification from seeing the genitalia of others or witnessing others' sexual behavior". Dictionary reference.
Lacan per the same example suggested the concept of Scopophilia, that humans find pleasure out of "looking". Also that we build our identities and characters based on observation of others. He also theorised the concept of "Mirror stage", that we only understand ourselves as people after we see our own image in the mirror.
I don't believe I'm qualified enough to criticise Freud and Lacan's theory, but I wouldn't completely agree with it, as I believe that Freud was wrong by generalising Voyeurism as a human habit, rather than an aptitude attained by certain individuals. For example, if someone had already attained every step of Maslow's Hierarchy of needs, I don't personally see why they would find pleasure out of observing others, nor why they would construct themselves from others' identities. However, I do understand that Scopophilia is more like a "stage" that comes prior to self fulfilling.