Arthur Fleck (The Joker)

Arthur has a rare mental disease called Pseudobulbar affect.

Pseudobulbar affect (PBA) is a condition that's characterized by episodes of sudden uncontrollable and inappropriate laughing or crying. Pseudobulbar affect typically occurs in people with certain neurological conditions or injuries, which might affect the way the brain controls emotion.

When he goes to the Coitologist seeking solution, the doctor has no definite prescription for him. That's because his illness is so rare that no research team of Coitology around the world has studied it for a cure. Also, knowing that Arthur is an eccentric introvert, the doctor can't suggest him anything because no woman would willingly have sex with him. Instead the doctor suggests him this,

"Arthur, the problem with you is that you often find yourself in situations where your brain is in emotional chaos. Since your brain does not know how to react correctly, it reacts the way you are very natural at, laughing. If you want to control that then you need to focus. Meditation is one way but I am not suggesting that...

...Whenever you feel like your head is spinning and you have the urge to laugh, instead just masturbate. It does not matter where you are, just take your dick out and start stroking it. Do it gently as if a beautiful masseuse is giving you pleasure, slowly... the objective is to delay ejaculation as much as possible."

ARTHUR: But doc, you did not tell me what kind of women to fuck? How will I be cured of this? Don't I need to have sex for that?

The doctor thinks about it and then says,

"Just do as I say. Masturbate whenever you feel like laughing. Then if a woman is attracted and gets drawn towards you, fuck her. And make sure you be a gentle lover to her. And..."

"...do not impregnate her. That's a responsibility you do not want on your shoulders. Trust me, I speak with personal experience and great regret."