Why do guys giggle around girls?

And i mean grown ups. Im in my early twenties and this guy is in his late twenties.

Everytime i pass by this guy at work, his friend gets all giggly. I didn't know them at first until i started talking to the giggly guy first. He seemed nicer. The other guy looked like a jerk but i got to know him and he's not. he's actually really nice. I only started talking to him because i was doing something and he came and started joking around with me. One day after that happened, he asked me if i can help him find a couple of things and i helped him. We talked a bit and he kept telling how smart i was.

A few days later, he came up to me and told me to help him decide what to buy and he asked me at what time i was off. So he said he would find me when he was off. When i would pass him, he would smile at me and i would smile back. And so he came to find me but i was extremely busy but he waited for me. He just kept watching me though. So i finally helped him and it was a bit awkward tbh. And he told me he had a girlfriend. Which is fine. Im not into him or anything.

But since that day, he has been kind of strange. He passes by but won't even say hi. He will linger around though. And at times, if im talking to my coworkers, he will pass by very closely. Other times he will just watch and smile. His friend still giggles a lot though. It all makes me feel awkward but i hide it. The other time i needed the computer he was using and he moved and offered me the chair but i said i was ok and he said "are you sure, homie?" I just laughed and said no im ok. And him and his friend just giggled and walked away.

This is weird. Im really shy but im one of the nicest people in the store. I talk to everyone. I dont know why someone would be like this with me. Could they be laughing at me or something?
Why do guys giggle around girls?
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