Why did Henri de Toulouse-Lautrec spend extended periods living in brothels?

Henri de Toulouse-Lautrec spent extended periods living in brothels because he was fascinated by the "life beneath the surface" of Paris after dark and sought to document the everyday realities of the people who lived there.1

Rather than attempting to moralize or glamorize his subjects, Toulouse-Lautrec aimed to depict the truth of their lives, capturing moments of fatigue, weariness, and isolation. He observed the actions and behaviors of sex workers and their clients with deep human sympathy, eventually becoming a trusted friend to the women who lived in these establishments. This immersion allowed him to create sensitive, insightful portrayals of their intimate surroundings and relationships, most notably in his 1896 lithograph series titled Elles.12