Its not a purpose that is put on us, I agree. But then again, I don't think we have a purpose, if we had I think we know it by now. But in my understanding of purpose, it has to solves or benefit something larger, whether that is positive or negative. If its doesn't then I wouldn't say that it was its purpose.
I don't see how anyone could disagree with thriving towards happiness is the ultimate purpose of life, given that we haven't gotten anything else. And if everyone can agree that this should be the purpose, then we ought to thrive towards it. Obviously the important thing becomes to identifying what is required for that and how to achieve it.
Oh a agree but as you say, there is no purpose