I'm a male, fair warning.
The answers here are great, but you should really read the book "Nice Guys Finish Last" It describes the phenomena people are alluding to perfectly.
I don't agree with all of the gender politics of it, but the premise is that there is a type of personality that craves the approval of women and sets up one-sided non-verbal contracts whereby their "niceness" entitles them to attention, validation, approval, love, sex, etc. Most of this springs from upbringing.
They struggle to say "no." They take on too many obligations. They self-efface ... what's worse, they assume everyone else should engage in similar matyrdom as well.
The "Nice Guy Syndrome" is a schema of victimhood whereby a man robs himself of his capacity to be a self-possessed, mature, capable and competent adult. It allows a man to shirk accountability for his choices and actions, assuming he doesn't get what he wants because he is "too nice," when really it is because he acts like a doormat who manufactures obligation and resorts to whining or tantrum throwing when he doesn't get his way.
Typically, a "nice guy" struggles to make male friends because of fear of competition or fear of same-sex intimacy. Instead he may have many more friends who are women who provide a degree of opposite-sex validation, though few respect him enough to desire him as a potential mate.
The antidote to nice guy syndrome is for the "nice guy" to strengthen himself emotionally and/or spiritually; allow himself to be rewarded and to reward himself (nice guys thrive on self-deprivation); form strong relationships with other men, especially father and/or brother(s); find rewarding pasttimes and hobbies, especially those undertaken with other men or that have physical, observable outputs; and treat his body and mind with respect (exercise, mental health care, good diet, avoiding excess intoxicants, etc.).
Breaking free of nice guy syndrome isn't about avoiding women. It isn't about hating women. It isn't about pickup artistry, or any other bastion of misogyny (though it often gets unfortunately roped in with all that). It's about the "nice guy" embracing himself and the full promise of his potential (rather than seeking out the validation of women at every turn).
In short, nice guys bend over backwards for validation and approval (especially from women), and they hold others accountable, rather than themselves, when they don't get it.
It does definitely exist.
I've seen my own life improve tremendously — especially in the romance department, but at work and among my other peers too — once I discovered how much I was living for the validation and approval of others (especially women) and took steps to strengthen myself and pursue my own goals more proactively.