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People look at people for a few different reasons. He might like you or be attracted to you but is too shy to approach. He may be trying to "read" you, he may want to ask you something, or he simply just may want to see what you are doing.
Just go say hi, talk about the weather, or just something simple and non-invasive at first, throw a little humor in. I like it a lot when girls come up to talk to me and I haven't talked to a guy yet who dislikes it. I always hear guys saying stuff like they think that girls who never initiate a conversation as boring or stuck up.