One issue that has long fascinated me is evolution. A few years ago, I read Dawkins' The Blind Watchmaker and was impressed by the way he connected evolution to other physical phenomena. If we think about things in terms of the probability that they should exist--this is highly speculative but bear with me--we would expect to find, for instance, things that last a long time (like rocks) or that are frequently produced (like clouds). But add in something that can replicate itself--but does so imperfectly--and the history of life on this planet seems like a foregone conclusion. (This is a partial response to creationist "arguments" concerning the improbability of life.)
I've also had debates with colleagues about evolution, trying to argue on premises like Dawkins' that natural selection (leaving aside the phenomena of mutation/variation and genetic drift, which are in some ways more straightforward, as generators of a kind of "randomness") is a general physical process not specific to biological organisms. Even higher-level phenomena like culture and technology, I would say, undergo an analogous process in their development.
As it turns out there are physicists who are trying to argue exactly this. A common "criticism" leveled by creationist idiots is that life is incompatible with the second law of thermal dynamics (viz., that entropy in a closed system increases over time). Contrary to what we might think, living things actually do more to increase entropy than do non-living things. One physicist in particular, Roderick Dewar, has even argued that what natural selection is really selecting for is not something vague like "fitness" but rather "maximum entropy production" (MEP):
Dewar has shown that MEP is the most probable behavior of an open, nonequilibrium system made up of many interacting elements, provided that system is free to “choose” its state and not subject to any strong external forces.... The large-scale state of MEP represents the largest proportion of the countless possible arrangements of the system's microscopic parts, regardless of what those parts are up to.
Natural selection in biology could work the same way, Dewar thinks: “In physics, to speak of natural selection is to ask, among all possible states, which is the one that nature selects.” This, he points out, is a question of probability. “The state that nature selects is the one that can be realized in more ways than any other. Biologists don't think like that, but I want to entertain the hypothesis that natural selection in biology works the same way, and see where that gets us.”
The issues here are actually quite complex, and I encourage you to read the whole article, which includes opposing points of view and an alternative theory about increasing matter and energy "flow"--did someone say "Heraclitus"?
Part of what we're dealing with is a question about the distinction between living and non-living things, but it also pertains to the issue of scientific reductionism: is biology ultimately explicable in terms of physics? Personally, I'm inclined to believe in emergent properties that require "higher" levels of description to account for.
In any case, I think it advantageous to see natural selection as a more general--dare I say metaphysical?--principle. Some have even argued that a kind of selection operates between different universes--although this is too speculative to entertain as anything more than a neat idea. When it comes down to it, we may not be able to ask why there is something rather than nothing, but the question of why this something rather than another may be in bounds.