There can be a few reasons why talking too much may be considered an issue. For example:
It can be disruptive to others, especially in settings such as meetings or classes where others are trying to pay attention or contribute to the discussion.
It can make it difficult for others to get a word in, which can be frustrating for them and can prevent them from fully participating in the conversation.
It can give the impression that the person who is talking too much is not really listening or is not interested in hearing what others have to say.
It can also give the impression that they are not able to control their impulse to speak which can be viewed as a lack of self-awareness or emotional maturity.
Over-talking also means it's harder to finish the conversation and move to a different topic, making it harder to bring variety in conversation.
In some situations, such as job interviews or public speaking engagements, talking too much can actually damage the person's chances of success by failing to make a strong and concise impression.
However, It's important to keep in mind that talking too much is a relatively subjective concept, and what one person might consider talking too much, another person might not. And in some situations, like socializing and networking, talking can be considered a positive trait.