For a long time, I wrestled with the question of why, other than divine fiat, to avoid acts like murder. I could argue fear of law, but I really don’t care about the law, even though I normally follow it.
Eventually, while driving home from work one day, it hit me: causing suffering is bad because suffering hurts.
Each of us knows, through accident or someone’s action, that suffering is an awful experience. We’ve all had a loved relative, friend, or pet die or undergone some other sort of emotional wound, whether it was a breakup or bullying or what have you. We all know what it is like to dislike pain, although there are some who know what it is like to like pain. (That’s why I use the term “suffering” specifically.)
Further, most of us have experienced what it is like to see someone else suffer. Normally, for most of us, the sight of someone we care about being in pain is painful to us. This is empathy.
Because we have empathy, causing suffering for someone else can also cause suffering for us. That’s the “me only” way of looking at it.
In any case, causing suffering really isn’t bad for any particular reason; it is bad by definition. Suffering is literally the term we have for everything we dislike experiencing.
So that’s the first half of my basis for morality: causing suffering is bad. The second half, with the same “by definition” argument, is simply: causing pleasure is good. The corollaries, obviously, are that preventing suffering is good and preventing pleasure is bad.
It is my contention that all of our moral decisions stem from an understanding of these simple principles. Fundamentalist Christians indoctrinate their children and prevent them from seeing outside perspectives to help them go to Heaven, a focus on preventing the suffering of Hell and causing pleasure. Doctors stick needles in children to prevent them from getting sick and to increase herd immunity, a focus on preventing suffering, even though it causes a little. Mothers deny their children cookies and force them to eat broccoli because they need proper nutrition to avoid suffering.
In a way, this is still the Golden Rule, or at least similar to it, because I am arguing that we all want pleasure and don’t want suffering. I’m not saying we should do this because some deity says to, though; I’m saying this is our genetic moral programming.
The Complete History of the Universe: A Personal Journey 
We have wisdom that resides within.
Indeed.
The Golden Rule is golden for a reason (and not, say, mauve–what would a mauve rule be like?) and I think it’s because we understand suffering and we have empathy.
I personally don’t like to cause suffering in any living thing (and sometimes even nonliving things that I’ve somehow managed to anthropomorphize).