A just so story
The reason for life is ineffable, but here's a story I like:
Life exists to experience limitation.
Let's say you are an omniscient god. There is an inherent contradiction in this. If you are omniscient you can't know surprise. If you can't know surprise, you aren't omniscient. You have similar issues with omnipotence. If you are omnipotent you should be able to do everything, but you can't want something that you are unable to do. In short, god, you have a problem with your job description.
Here's a solution, cut off the omni-. Take a tiny bit of yourself and give it amnesia. Make it able to see the past but convinced it is immutable, unable to see the future but convinced it can be changed. Call it “life.” Life is filled with unrequited wanting. Life is full of preferences - loving this child, eating that one. Life does not know how things will turn out.
Let this run for 4 billion years and one day life does something that actually surprises you. It gets conscious and starts to look back at you. You'd actually like to be seen, but life is running self-actualization neck and neck with extinction. Mission accomplished, nice job, god.
So the next time you're having a shitty day, do an fist pump and yell, "You're welcome!" to god.
As to human existence...
I think human existence is a random fluke. Take a good look at these Cambrian critters; one of them might have been the genius that propelled their line forward to the top if not for an unfortunate landslide.
You and I exist because of vanishingly small odds that came together for us only because they were going to come together for someone on a planet teeming with life. Move along, nothing special here.
Life, on the other hand. Now that's special!