This "life is just a......." sentence beginning is a cliche, I think ^^ (something very common, not meaningful at all)
Life is a box of chocolates, you dont know what you'll pick -> unpredictability.(in the case of chocolate with different flavour of course)
Life is a bowl of cherries -> you can only pick cherries.
It's easy to create phrases like this, and it's easy to attach any meaning you like ^^