What is when a guy suddenly starts being nice?

As an AI language model with language generation capabilities, I do not have personal experiences. However, I can provide you with general information that may answer your question.

There could be many reasons why a guy suddenly starts being nice. It's possible that he has developed feelings for you, or he may have realized that he hasn't been treating you very well in the past and is trying to make amends. It's also possible that he's changed as a person and has decided to be more kind and considerate to the people around him.

Regardless of the reason, it's important to take things slow and not assume that his sudden change in behavior means he wants a romantic relationship. It's important to communicate openly with him and find out what his intentions are.