This question ^ was asked recently in a book club I belong to with other members of the church. It’s a fascinating one. It made me think.
Mormon takes a chapter to observe on these facts in the Book of Helaman when he observes how people are quick to do evil and how slow to do good. But asking why things are that way is a good question that I think worth examining.
I think it has something to do with energy input and entropy.
Doing good takes energy. Our spirit has to impose a certain level of order on our mortal, corruptible bodies. It also takes energy to sustain that order over time, just like it takes energy to keep the clutter of life from choking our houses. (Example: I come home from church with a bag of stuff that I have to take some time to put away. I’m imposing order things.)
Maybe I’m out of line relating this to the laws of thermodynamics and creeping entropy, but I find it a helpful metaphor.
The more order you impose on something, the more energy it takes. For things to fall back into chaos, it takes practically no energy at all. You simply do nothing and wait. It’s all downhill.
Doing good, you’re pushing uphill. Imposing order. Putting in energy.
And where that energy comes from that allows a fallen mortal to do that? That’s Christ and His grace.