Why would a guy stare at me while talking to his friend for like 5-10 minutes straight but never say anything to me?

So there’s this guy at my work who I seem to catch staring at me/glancing at me a lot, but he never comes up to talk to me. When I look back at him, he sometimes suddenly looks away. Today he did the staring thing again but for like 5-10 minutes straight while he was talking to his friend and then again when I moved to go help our coworker with something. Why would he be staring at me and never approach me? Is he maybe too shy or something?

Why would a guy stare at me while talking to his friend for like 5-10 minutes straight but never say anything to me?
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