He flirts,you ask him out, he said no, but why does he still flirt?

Is it a general rules for a guy to still flirt with the same girl he said no to going out with ? Is there a reason why this would happen ? I have a guy still flirting with me even though he said no to dinner or just hang out when I asked him This started several months ago when I realized one of my male coworkers was flirting with me . ( I had just got over a major breakup and had not notice it until then) . I thought maybe he was just a kind of guy who was just a natural flirt . I never though anything of it until by chance I notice he changed his appearance . After I made a comment that he looked better with a goatee then a full beard . Thinking maybe he like me a little I asked him out to dinner he never really answers me(As he walk away I say is that a no and just nodded his head . ) To save face I later asked him if we could hang out as friend he said he was not interested in being friends . So I started to ignore him thinking I may a major mistake . However he still showed all the signs that he is into me . And even goes out of his to make sure I know he is around . He even point out that he got his ear pierced with the same type of ear rings I said would like nice on him . Should I give this some more time to develop or walk away ?