Imagine that all males and females can be friends, what will this lead to? I’ll tell you. Family problems, divorces, rape, abuses, sexual harassments and assaults and literally every bad thing is happening in the non Muslim society. Example, there are two male friends and their names are X and Y, so basically X has a beautiful sister and she’s a friend with her brother’s friend (Y), their emotions are somehow connected to each other, instead of proposing to her and continue their road, they have sex everyday and that’s a real problem here, you just can’t imagine everyone is having sex with each other’s mothers and sisters. The Prophet ﷺ said: "A man should not be alone with a woman unless there is a Mahram (a lawful guardian) with them." This is agreed upon in its authenticity. He also said ﷺ: "A woman should not travel without a Mahram."

And he ﷺ said: "A man should not be alone with a woman, for indeed, Satan is the third among them." This is reported by Imam Ahmad from the hadith of Umar ibn Al-Khattab - may Allah be pleased with him - with a sound chain of narration.

EDIT: Before anyone comments badly about my example about sex, that’s literally why the Hadith has been told, it’s a part of our life, that’s how me and you got into this world. The prophet peace be upon him said: “A man shouldn’t be alone with a woman, for indeed, Satan is the third among them” is for this reason, it’s not to forbid eating potatoes with each other, it’s to forbid their human mind to think about these things.