Why a mother of a two month old feel that her milk is not enough for her baby?

Its very common for a mother of two or three month old infant to feel that her milk is not enough because:
1.she feels so.. due to her perception of baby size or compared to peers
2.other family members or friends feel so
3. Baby appears thin.
4. Baby cries often
5. Baby leaves breast in few minutes
6. Mother feels she has less out put
7. She feels her breast size is small or not enough

All these reasons are incorrect.. if mother milk is not enough, the baby will start losing weight. Now why these assumptions are incorrect? Dont hesitate to ask here.
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