Having written about reason and rationality recently, here's some thoughts that came to mind (weird phrase, since where else would thoughts appear?) as I was musing about how people believe in God's will and karma as other-worldly explanations for why things happen as they do.
It struck me that God's will and karma are appealing notions for many because they provide reasons for happenings that often aren't pleasant. Or are surprising.
For example, I know someone who got stage 4 lung cancer even though she never smoked. Thankfully, she's doing quite well, even after several years have passed since the diagnosis. She isn't at all religious. But if she was, probably the idea that her cancer is in accord with God's will would comfort her.
"Will" implies a reason behind it.
Otherwise we'd speak of "God's caprice" or "God's random choice." When someone wills that something happen, even if that someone is a supernatural being, seemingly there's a purpose that will is intended to achieve -- even if that purpose is unknown to us.
Likewise, karma definitely entails reason. Typically karma is viewed as encompassing more than one lifetime, though the word also can be limited to causes and effects occurring within a single life. Good karma is the result of positive actions; bad karma is the result of negative actions.
You know, the whole as you sow, so shall you reap thing.
Thus karma is nothing but reasons. Those reasons for why something is happening to us may be lost in deep time, if we believe in rebirth/reincarnation, but they exist nonetheless. At least, that's the way karma is conceptualized.
( I wrote a book about karma, "Life is Fair," so I'm familiar with the term.)
Obviously there's another way to look upon the reason something is happening: as being in accord with the laws of nature. The benefit of this perspective is that it is scientific, not religious; factual, not fantasized; coherent, not confusing.
As with God's will and karma, often it is very difficult, if not impossible, to discern the precise reasons that produce a happening. In part this is because chaos theory, though deterministic, says that initial conditions can play a big role in an event.
Often this is called the "butterfly effect." A butterfly flapping its wings in Brazil can theoretically create an atmospheric instability that, over time, is magnified into a hurricane striking the United States.
Even so, the beauty of embracing laws of nature as the reason things happen as they do is that no outside concepts are required. No need to believe in the unseen will of an unseen God. No need to believe in the unseen force of karma extending across multiple unseen lifetimes.
We know about the laws of nature through science and everyday life. They are the manifestation of nature's reason. Sometimes they even can be described in mathematical terms.
So if you're looking for a way to understand why life here on Earth is as it is, there's no need to look beyond three simple words: laws of nature.