Homosocial bonding is different between men and women, and this causes a lot of confusion and why it's assumed that men and women cannot be friends.
Men see their friendships as camaraderie, hanging out, occasional complaining, and chilling. There's plenty of support mechanisms in place, but they're not intimate, per se.
Women on the other hand are intimate, affectionate, they talk more about how they feel than how things happened. The support mechanisms are explicitly intimate.
So, men who are not used to intimate physical contact and discourse have to translate a person of the opposite sex being physically and emotionally intimate beyond the boundaries of relationships he is used to.
Women may say that this is really the man's fault for getting his hopes up, but it's worth noting that scientists have found that this disparity in relationships has an actual significant effect on different genders. Men and women are programmed differently.
As the study shows, women share their emotional intimacy through much broader networks. They don't restrain it for that one special person. They give it out freely. They take it in easily.
And also, as the study shows, men reserve their emotional intimacy for one special person. They rely on that person. They hold back until they find someone they can trust and then pour it out to them.
This actually makes male intimacy a far more dear thing than female intimacy. This is why men "overreact." This is why men panic. Above all, this is why the Nice Guy misreads his interactions with a woman he likes.
Of course, this has an interesting side-effect. To wit, when bad relationships end and men are single, they actually do better emotionally than single women do because what a man derived from the relationship had a higher cost for him. Men don't mind being called "single," what they mind is having their only intimate outlet being in jeopardy or, worse, being turned against them, such as in a bad relationship.
EDIT: fixed the wording of the above.
In opposition to this, when women are single, they actually do worse than when they are in bad relationships. A woman in a bad relationship still has her emotional network intact. A woman who is single has instead had her relationship status changed.
Now you can look at the nice guy phenomenon through a sharper lens. Men are used to emotional intimacy being saved for a special person, women are not. Women find emotional stability in poor romantic relationships, while men do not.
This confuses the fuck out of the Nice Guy. None of this makes sense. A woman is being emotionally intimate with him, he thinks he's special since that's how he would act. A woman stays in a bad relationship, he thinks it's illogical since that's not how he would act. Combined, this becomes the "I would be good to you, what's wrong with you!" mindset.
Of course, he doesn't understand that a woman has cultivated many intimate relationships with friends and family while he has been working on the one trying to develop a romance. The woman doesn't need to get her emotional support from her romantic relationship. In fact, she can spend all day talking about how bad her relationship is -- but at least she's not single, and maybe her partner provides some other value beyond emotional intimacy.
In regard to how the nice guy is viewed, the woman sees his actions as those of just another friend, since that's how she would act toward her friends -- freely intimate, physically affectionate. When the guy doesn't get what he wants, she will sometimes feel betrayed, primarily because he has willingly integrated himself into her network and then has destroyed the status quo.
Ninja-Edit: It's worth noting that there become unspoken non-rules about flirting and relationships. If a man is physically affectionate with multiple women, it's probably safe to befriend him since he probably isn't going to balk at his intimacy not developing into romance. It also probably means that he's not going to respond to romantic advances as quickly. Interestingly enough, this may all be at the root of our inculcated romantic steps. Men are usually non-intimate, so intimacy means romantic interest. Women are usually intimate, so sexual interest means romantic interest. Oddly enough, there is still an expectation for men to make the first move in such an environment.
Over the years, a mix of misogyny, misandry, entitlement, and sheer ignorance and indifference to all parties involved have turned this issue into a point of contention. Everyone is trying to translate it through a universal precept of human interaction, often ceding to one side or the other points they have not actually managed to make out of politeness or self-loathing or whatever.
As you can see, men get over it faster than women do. Not universally, of course, but men are more comfortable being single than women are. This is where the myth of "commitment-phobic" men comes in. For men, advances in one's relationship are emotionally expensive, each step more costly than the next. For women, it is effectively a status change.
Like all studies, generalizations are merely a recognition of the trend in a group. Personal anecdotes and asides are all well and good and I am not trying to discount them. That said, the trends are apparent.
TL;DR Male platonic relationships are friendly camaraderie, female platonic relationships are intimate and physical. When men try to be friends with women, they sometimes misinterpret each other's intent and feel betrayed when things do not go as expected.
This also has an effect on initiated romantic relationships as well, since each partner is investing and seeking something different in each stage.
But at any rate, there's no sure-fire rule about when to message a girl. If she's into you and you phrase your message right, then it doesn't matter when you message her.