Why is it forbidden for a man and a woman who are not mahrams to have a relationship?
Question

Why can't a Muslim date?  I'm an extremely strict Christian but I have Muslim friend who I'm trying to understand.
Praise be to Allah.Islam forbids a man to be alone with a non-mahram woman (one who is not his wife or a close relative) even if he is teaching her the Quran, which is the Book of Allah, That is because the Shaytaan (Satan) would come between them. The Prophet of Islam (peace and blessings of Allah be upon him) said: “No man is alone with a woman but the Shaytaan will be the third one present.” If this woman wants to hear about Islam and read about it in detail, she can look at books about Islam that have been translated into different languages, so she can take the language that she understands, then if she understands what attracts her to Islam, she can become Muslim. If she does not understand something and wants someone to explain it to her, it is permissible (for her to speak to a man) so long as there is no khulwah (being alone with a member of the opposite sex) involved – so she could have a mahram (close relative) with her, or a group of women, and the man should be a trustworthy Muslim, or a group of trustworthy men who could sit with this woman and teach her about Islam so that she can understand it and proof be established for her. This is permitted.
Shaykh ‘Abd-Allah ibn Jibreen
Allah wants to make the Muslims pure, hence He forbade them all means that may lead to evil, immorality and obscenity. You know that if a man is alone with a woman and starts a relationship with her, this relationship often leads to bad consequences, and that being alone with a member of the opposite sex is the way to immorality and fornication. It is not permissible for a man to praise himself and say, “I will be not affected by being alone with a woman.” Islam does not allow the opportunity for things to get out hand; it keeps people away from the steps that might lead to that in the first place. The rulings of sharee’ah were revealed for all people, it is not the point that there are cases of khulwah which do not lead to haraam actions such as touching and kissing. Why should a person expose himself to temptation?
Is it not the truth that if a man is alone with a non-mahram woman and there is no one else present, that something may cross the mind of either of them, even if nothing actually happens. But frequent meetings may indeed lead to something happening.
In this case, sharee’ah closes all doors that may lead to evil.
If a woman needs to speak to a man for a genuine reason, or vice versa, then that may be achieved by sending letters, without having to meet, or they may meet from behind a screen, or in the presence of others so that no khulwah will be involved. This is provided that both are modest and wear concealing garments.
And Allah is the Guide to the Straight Path.