Let's say that a man and a woman are friends. They have known each other for a while and nothing has ever happened between them. By "nothing," they mean that they've never kissed.
The chemistry between them is palpable and they've been in situations where he could have made a move but he did not. And she recognizes this and respects him for it. She knows that his intentions are ostensibly pure.
She may or may not find him attractive, but it doesn't matter. He's a good guy and she likes talking to him. And even if he's cute, she's not interested in him like that. Or so she thinks.
On the flip side, he thinks that she is sexy. He'd love to go out for drinks, take her back to his place, inch closer to her on the couch. But she's made it clear that she's not looking for a relationship or she's already in a committed one. He's a respectful guy and completely understands. He's been placed in the friend zone but he's not bitter about being there. In fact, he embraces the opportunity. He continues to message her because she's a cool chick.
And, honestly, when a hot girl messages you, it's impossible to ignore her. Especially when she's a nice person.
Because of his sincerity, she's comfortable enough to share her secrets with him. Stuff that she's not willing to tell her significant other, perhaps because he is not quite as understanding. Or maybe, it's bad stuff about her boyfriend. And you know what, it doesn't matter how independent a woman is--when her boyfriend messes up, she needs someone to talk to. More specifically, she needs to talk to a friend, someone she can trust.
Similarly, when his girlfriend is acting up, he will need an outlet. He'll need someone to be there for him. And you know what's remarkable... he's willing to have an emotional conversation... knowing that things won't get intimate. Over time, he became the Michael Jordan of the friend zone. He's not going to make a move because he doesn't want to jeopardize the friendship. It means too much to him.
Typically, women tend to be drawn to guys with good personalities. Yeah, of course, looks play a part. But as she continues to laugh uncontrollably at his corny jokes, as she continues to turn to him for his advice, as she continues to feel secure in his presence, she will begin to let down her guard.
On the other hand, men tend to be drawn to women who look good. He was first attracted to her because she had sex appeal but as he got to know her more and more--she became beautiful. Controlling his feelings will become that much more difficult. And when he is vulnerable, he's no longer in control.
The truth is, they may never do anything physical in life. They may never cross the line. But a relationship can never be truly platonic if you have to set up boundaries. A relationship can never be truly platonic if you have to adjust your feelings. A relationship can never truly be platonic if you have to pretend that you are happy with the way things really are...when deep down--you want something more.
And for some friends, they may eventually cross the line. Because, ultimately, we are human.
Disclaimer: If there is absolutely no physical attraction between a male and female, then, I would say, it is possible that they can truly share a platonic relationship. But once a man is attracted to a woman; or the woman is attracted to the man; or both are attracted to each other--the relationship cannot and will never be platonic.
You can, however, pretend that it is platonic. And for some people, this is good enough.