What keeps a guy from approaching you, if you believe they may be into you?

So at work there is this guy who I noticed would always steal glances at me whenever he sees me or if I am walking somewhere and he's around he would find a way to walk behind me. I want to ask him myself but since we don't really talk at all, I don't want to assume anything.

What keeps a guy from approaching you, if you believe they may be into you?
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