If there is no "inexorable trend toward more advanced forms of life" why did more advanced forms of life evolve with increasing velocity ever since the appearance of life some 3-3.6 billion years ago?
Firstly, the vast majority of the Earth's biomass continues to be relatively simple organisms. It's just that you don't tend to think about them. (Just stop and ponder, for a moment, how many individual bacteria there are on planet Earth. Whew!)
And think about it this way. Imagine the first living thing. It's literally the simplest organism that could possibly still be considered alive. Actually, imagine a population of them. Now, imagine that one thing changes about a few of them.
If that change was for the simpler, they all die, because they're no longer complex enough to be alive. If the change was for the complex, they live, and have advanced capabilities or something.
When you're at the bottom of the barrel, there's no place to go but up.
If you see a "trend" in evolution towards complexity, you're looking at it the wrong way. Complex, multicellular life is like a blip on the evolutionary radar. Most living things continue to be very simple indeed. It's just that it's easy for humans to forget about them.
Increase in complexity seem to me an observational fact.
Then you need to look a little harder. I'd reccommend reading Gould's "Full House". It's a great explanation of thise very phenomenon.
More importantly, why did life first emerge?
Why not? It's just a kind of chemistry. I'm inclined to think that it's inevitable, given enough time.