Answer:
1. It's because when they talk to girls (Not all), they fill their head with all sorts of doubts and worries and then feels nervous as a result.. like for example they could be thinking "Am I making the right impression? Am I too annoying? What does she think of me?"
2. It means that they think that person is pretty. But if it gets too weird or annoying, you should report/tell an adult about it right away.
