I came across this quote from the movie *RocknRolla*:
> Oh, beauty is a beguiling call to death and I'm addicted to the sweet
> pitch of its siren. That that starts sweet ends bitter, and that which
> starts bitter ends sweet.

I understand the metaphor. But I'm confused with the sentence structure of

>That that starts sweet ends bitter, and that which
> starts bitter ends sweet.

Are the first *that* and the *that* before 'which starts bitter ends sweet' conjunctions while the second *that* and *which* relative pronouns?

If so, what are the two relative pronouns referring to?