The fact that there is no absolute does not mean that one purpose is just as good as another.
It does if we are talking about an objective purpose. We can't come up with a reason for one purpose being better than another without begging the question.
As follows:
"We ought to help babies."
"Why?"
"Well, you were a baby once, and somebody helped you."
"So? I'm not a baby anymore. Let them look after themselves."
"They can't look after themselves."
"So, who cares? Let them die."
"But it's not right for you not to care."
"Why not?"
"We should help each other."
"Why?"
And on and on. Eventually we get to the point where we have to just utter a tautology: "We ought to help babies because it's right, and it's good to do what is right."
We haven't advanced our argument at all. We might as well have just stuck with the initial statement: "We ought to help babies."
The universe may be arbitrary and existance itself may be no more significant than non existance. But the fact that there is something rather than nothing is cause enough to celebrate life and know that for all our ignorance of why or how we still have the knowlege of being.
I understand what you are saying, and I would agree that it is impossible to live nihilistically.
But why is the fact that there is something rather than nothing cause enough to celebrate life?
There is no answer to this question.