because if we weren't, we wouldn't be.

Because Reddit banned us all?

Because our parents fucked.

Enough rocks slammed together billions of years ago and now we have to pay taxes and watch corporations ravage the planet in the name of shareholder growth.

I am here because my parents thought the only ethical way they could have sex was to get married and have children. They had no interest in actually raising those children. I have been suicidal all my life, but at least I am finally far away from them.

why are we here on earth??

Because the other planets are much less hospitable.

There's nothing special about us, about Earth, about our situation except that which is self-evident: we're here, Earth supports us, and we can ponder the universe, but that really might be all there is to it. It's dangerous to attribute any reason to it, because reason implies purpose and purpose implies oversight. Or, as Douglas Adams put it:

This is rather as if you imagine a puddle waking up one morning and thinking, 'This is an interesting world I find myself in — an interesting hole I find myself in — fits me rather neatly, doesn't it? In fact it fits me staggeringly well, must have been made to have me in it!' This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, frantically hanging on to the notion that everything's going to be alright, because this world was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise. I think this may be something we need to be on the watch out for.

buncha molecules randomly bumped into each other in a way that allowed them to repeat infinitely.

To crush your enemies and see them driven before you and to hear the lamentations of the women.

"is it happiness or sufferings?"

Or maybe it's just a big cloud of gas, compacting into mass, exploding and growing into a universe. Maybe we think we are so incredibly grant but we're just part of a historic cause and effects chain that's still going. Maybe we want a reason but there is none and what we really want is the feeling of making a lasting impression on the world, or having something to work towards. But it's all just happenstances and probabilities.

I'm sure you can find a thousand answers but never underestimate the power of Occam's razor if you wanna find the most probable one.

why are we here on earth?

Well, it's the best of all planets, after all.

We just are. No reason in particular.

But since I'm here and there was a lot of time where I didn't exist yet and probably will be a lot of time when I won't exist anymore I might as well make the best out of the time when I am here.

Because I said so.

We get to decide that for ourselves. It might be for selfish or selfless purposes that we decide, but the decision remains in our own hands.

We're food for the gods.

I have no idea. But evolution threw us quite a curveball when it gave us the ability to ponder.