1) It's a fallen world... that is, Human nature is flawed, always has been always will be.
By flawed I mean, we engage in thoughts or acts that we know we shouldn't do, and don't perform those things we know we should.
That will sound very familiar to some of you so I need not cite the source, except to say it was known long,
long before the first century AD.
**
You can attribute it to an inherent spiritual flaw, or to how homo sapiens developed through eons.
But which ever attribution you choose, we are stuck with it.
The Jews of
the Old testament understood it. The Stoics of the Pre-Christian Roman Republic understood it.
Homer certainly understood it; read the
Iliad and/or
Odyssey. The Seven Deadly Sins are on display in all their corrupting, yet oh, so banal manifestations.
Most of the world religions have also comprehended fallen human nature, so that goes quite a ways back also, several thousand years BC .
**
2) In recognizing , and more importantly, accepting (1) you can see that
humanity hasn't made a whole lot of spiritual/moral/ethical progress from at least the time when
men first learned to write, to make permanent records of their thoughts, their observations of their fellows, their histories.
3) So if you can accept (1) and (2), then you know you are going to have lifelong, uphill battle against your worst inclinations.
Forewarned is forearmed....it's a real battle and victories are hard-won.
A character in a contemporary comic strip knew this: "Why is the right thing to do so often the most difficult thing to do?"
The converse (which is what the OP is asking) is: Why is the wrong thing to do so often the easiest thing to do?
** For those interested or even just curious about how far back this was recognized, read the CS Lewis essay: Illustrations of the Tau.
The Tau : Lewis' name for
the objective rational and moral order embedded in the cosmos and in human nature.
Other names for it include Natural Law or Traditional Morality. Lewis borrows the term from Eastern religions for the sake of brevity and in order to stress its universality. (this definition from several website on CSL and
The Tau)
It is in a larger work
The Abolition of Man, (yet that little volume is not large at all; my hard copy is 61 pg, and that includes the appendix containing
Illustrations of the Tau.)
I recommend reading the entire "
Abolition..." ...but for those who merely want to "cut to the chase" of Lewis'
Tau, go
HERE 
