My parents always told me it was because of a three-in-one god and that 7 was "God's number" - probably because it came up so much in the Bible, though on that reasoning I don't see why 12 wouldn't be lucky. That was also why 6 was supposed to be unlucky, because it was associated with the devil. So I suppose it just depends on people's personal idea of why a number is lucky!

In Chinese, numbers are lucky or unlucky based on the way they're pronounced and what other words that syllable sounds like. So four ("si") is similar to death ("si") and you should never give someone four of something as a wedding present!