The chocolate box of life

I've never really understood the meaning of Forrest Gump's quote of "Life is like a box of chocolates, you never know what you're gonna get" I mean I always know what I'm gonna pick before I put it in my mouth. But the following quote in the Likely Lads film pretty much summed up the whole film and it's a quote I personally say when things aren't going my way...

"In the chocolate box of life, the top layer's already gone... and someone's pinched the orange cream from the bottom"

Hilarious, but so true.