Because they are showing off to other boys and showing just how immature they really are. A mature person doesn't need to brag.
They probably like the new girl, or make fun of them.
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Some guys may brag about new girls to boost their ego, gain social status, or seek validation from others. It could also be a way for them to show off and attract attention. However, not all guys engage in this behavior and it's important to remember that bragging may stem from insecurity or a need for external validation.