The “nice guy” is a cultural staple. Most women I know have been on the receiving end of this phenomena, where a man (or woman, though the vast majority of these people are men), is at first overwhelmingly nice to a person only to turn upon them once it is established that the attractions they feel are one-sided.

Most men in this position respond in anger and embarrassment. Some demand recompense, and so on.

There are many problems with this, so let’s break them down.
Starting with the core of it: They are handing away control of their well being to another human. This made doubly bad when you consider that the person who is being handed this power probably didn’t want it. Societal politeness might force them to play along for a while, but there comes a time when the person will begin to resent having to be in charge of someone else’s emotional well being.

Thus they reject the advances of the “nice guy” in effect they are handing back the reigns of control to him. He, having given up agency over himself, responds to this perfectly normal situation as if he’s been judged harshly and unfairly. He responds with what he thinks is righteous anger. When his anger is rebutted his shame feeds this anger, and so a toxic feedback loop is formed.

The average “nice guy” cannot see that the pain he’s feeling is self-imposed. He’s externalized his happiness unto an object, (most nice guys see women as objects) and when the object fails to meet his high expectations, he feels slighted. Thus he avoids self-reflection and growth.

Dating, careers, and achievements are the main focus on many a human’s life, and these are exceptionally important to a healthy life, but so many people hand away their ability to be happy to others who never asked for it.
The nice guy fails to see that attraction is frankly not that romantic, though romance is important to the act of dating, the spark of attraction is largely subconscious. A smell, a face shape, how they smile or talk, these things trigger attraction, and cannot be influenced really.

More importantly, he pins his self-esteem on some external measure, and assumes, often without merit or evidence, that he’ll be successful, so when he is not he cannot tolerate the tension between what he thought the world would be like and what it is like.

So what can be done?

The first step begins with removing your worth from the external world. This isn’t easy. There are all kinds of social games we have to play to be successful, but with enough training, you begin to be able to hold in your head that the social and societal games are just that games.
When you find yourself in a moment that you find yourself upset, if you can, and it will take practice, try and take a breath, but do not try and calm down, but instead take the role of an observer. Observe where you’ve put the locus of judgment. Is it external? Are you expecting the world to give you something? What is going on here?

With time you will be able to see the partners that led to the externalization and begin making conscious choices.