i) Why assume that there's a purpose of life (or a purpose to life) at all?
ii) Why should there only be one purpose (one meaning) to life?
“Do you want to know what the purpose [meaning] of life is?”
And the answer will usually be a religious one; though sometimes it may be “spiritual” or have something to do with what the questioner himself thinks the purpose/meaning of life is.
There must be a primary thing from which everything else owes its existence.
However, where does purpose or meaning fit into that belief? We could all owe our physical existence to the First Cause or to the Big Bang. However, there's no built-in purpose or meaning here unless it's argued for; rather than simply assumed. (Most biological things owe their existence to air and water – are they “primary things”? Is the purpose/meaning of life air and water?)