machine learning - Overfitting and Underfitting - Cross Validated

I have made some research about overfitting and underfitting, and I have understood what they exactly are, but I cannot find the reasons. What are the main reasons for overfitting and underfitting

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Is it possible to overfit on a very small training data? - Quora

K-Fold Cross Validation Technique and its Essentials

K-Fold Cross Validation Technique and its Essentials

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Overfitting and Underfitting with a real-life example - Shiksha Online

Overfitting: what is it? How can I avoid it?

machine learning - Overfitting and Underfitting - Cross Validated

Underfitting and Overfitting Explained Newbie

Overfitting in Machine Learning: What It Is and How to Prevent It