In broad strokes, yep.
() I say we think we know because I'm actually agnostic (I typically say 'atheist' because it stops some conversion attempts I'm not looking for from even starting, judging by your two comments on this thread I reckoned I can give you the honest answer as you won't try to convert me into anything!) and I believe we can only know what we conclude from the information collected by our senses and our thinking process after that, but an error in either the information collection (think of how the first scientific estimates of the age of the planet were off by a long shot on account of choosing a poor thing to measure -- I'm thinking about the work of Halley here) or in the thought process would result in bad knowledge, and it seems to me there's a part of the scientific-minded population right now that has a blind spot for this: there's overconfidence in science.
To cut to the chase: our brains are just piles of chemistry. There is no meaning. We make it up, and that is ok. “Why” we exist is coincidence and millions of years of happy little evolutionary experiments blindly conducted by nature. Maybe there is a Deus ex Machina in there, but for our purposes does it really matter?
It means (pun intended) that the meaning is not self sustaining term in our reality - there is no absolute meaning outside of our perception. There is only relative meaning (as in what kind of meaning my perception assigns to things that can be observed by me - simply put - we create our own meaning).
It can be simplified further to there is no meaning or the meaning does not exist but in my opinion this is oversimplification and reeks of nihilism.
So if You look at things from the point of view described above the "why" question (which can be paraphrased to "what is the meaning") is wrong.
I also wanted to add that it's not that I think it is wrong to ask such questions, only that I think they're wrong when considered from within our system of knowledge, so I find them unanswerable.
Kind of how like "what kind of food does that digital computer like?" is a wrong question.
I'm not religious either, but I always find this hard to believe. What are the odds nature happened to provide all the building blocks for us to be here to question our existence? It seems far more likely that (i) nothing would exist, (ii) the universe wouldn't have the right combination of properties and forces to maintain its own existence, or (iii) it would be a boring universe filled with a couple of basic elements capable of producing nothing of interest. Instead, we have complex life and we're here building iPhones and spaceships.
For that reason I can't believe there's a single universe and through coincidence it happened to contain everything needed for life. Even if we go with the multiverse theory and a near infinite number of universes, I still find it difficult to believe. You can argue the universe is filled with a bunch of garbage and we're assigning meaning and value to that garbage because it's us, and we want to feel important. However, I really don't feel like anything (and certainly not something as complex as us) should exist in the first place. I want to say it's too much of a coincidence to happen by chance, but at the same time, I don't have a better answer as to why we're here.
What is so remarkable about the iPhone or the spaceship? Why is it worthy of note when compared to any other phenomena in the universe? What brings you to make a distinction between a live human body and an inanimate celestial body?
I'm not trying to say that humans are more important or meaningful than a rock. I agree with you that nothing inherently has meaning and it's an attribute we create and assign. I'm only saying that we're intelligent beings that are capable of some rather advanced tasks, such as creating a iPhone. In my opinion, it seems far less likely for us to exist than either nothing, or a simpler universe without us.
Yes, it's not impossible, just like I could throw a handful of sand in that air that falls to the ground and happens to write the story of my life. It's so unlikely though that I can't help but wonder if it wasn't just chance that we're here.
The why question has no answer nor will it ever have an answer. Life doesn't need justification or orchestration. It's a freak accident of molecules. It could have never happened and it can end by means of a disaster and the universe will happily continue without it.
As to what a human can/should to with their life other than not dying and trying to reproduce, that's an entirely cultural question. Cultural is a fancy word for: we made it up.