The Messenger of Allah (sal Allahu alaihi wa sallam) said: “When a non-Mahram man and woman meet in privacy, the third one present is Shaitaan.” [Tirmidhi]
We know that Shaitaan’s job is to lead people astray. When a man and woman meet in privacy, he is there to emotionally excite them and to invite them to engage in unbecoming conduct. This is why Allah (subhana wa ta’ala) has prohibited it. It is necessary to emphatically follow this prohibition. Even elders, teachers, doctors, mentors and cousins should strongly avoid being with non-Mahrams in privacy. Doing so is sinful.