In the traditional culture of China, rain represents wealth and affluence, and crossing one's room may be considered as a symbol of missing money. In addition, rainwater flowing into one's home may also be considered as an ominous sign, because "water" represents "wealth" in traditional culture, while "flowing away" is considered as a bad symbol.
Therefore, if the rainwater from the neighbor's house flows to his own house, it is suggested to negotiate with the neighbor, such as building a wall between two houses, or using other methods to prevent the rainwater from flowing to his own house to avoid these bad phenomena.