How about that: Why should there be a meaning of life at all? I mean, searching for those kind of answers is a common humanly mannerism, sure, but who said it's generally necessary? Maybe existence itself is not even relevant. Perhaps life - as we know it - is just a trivial by-product of something bigger (or let's say "more meaningful" in this context, :D), for which mankind will never have the capability to understand it.
Sure you can define what's your very own mission on this planet, but that kind of meaning is just an individual one and barely transmissible to a generally context. So must there be an overall meaning of life for all human beings? I don't think so.