There are several reasons why a guy might look down when you pass him. It's not always easy to definitively know his intentions, but here are some possibilities:
Possible Reasons
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Shyness: He might be shy and uncomfortable making eye contact, especially with someone he finds attractive or interesting. This is a common reason cited in various online discussions (Quora, GirlsAskGuys). https://www.quora.com/Why-does-this-guy-look-down-when-I-pass-him https://www.girlsaskguys.com/flirting/q80070-guys-do-you-ever-on-purpose-look-down-after
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Attraction and Insecurity: He could be attracted to you but lack the confidence to make eye contact. Looking down might be his way of coping with his nervousness or feelings. This is another frequently mentioned explanation. https://www.quora.com/Why-does-a-guy-I-like-look-down-every-time-I-pass-him
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Neurological Differences: In some cases, looking down could be related to neurological differences, such as autism. This is a less common but important consideration. [Reference: Looking down could mean any number of things. It could mean he is shy. Or, he finds you attractive, but lacks self-confidence. Or, he might have some neurological differences, such as autism.16-Jul-2016]
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Other Factors: There could be other reasons unrelated to you. He might be preoccupied with his thoughts, sad, or simply tired. An example could be someone lost in thought and not even noticing you.
Conclusion
Without more context, it's impossible to know the precise reason. However, shyness and attraction are common possibilities.
