In some people, messiness is due to literally not seeing stuff. While some brains hypernotice (low latent inhibition), others hyperfilter.
In others, it's due to fatigue or overwhelm. Long days at work + commute, or baby care, etc.
In others, it's due to more stuff than is actually manageable.
In others, it's due to a preference to be surrounded, a sensory comfort in that.
In others, it's due to not having an easy, close place for each item (should take literally three seconds to put away).
In others, it's due to anger at a roommate/spouse.
Several of these are easily resolvable, if people agree they want them to be.
The first one can be taught, for example, in one of these ways:
1. Like Axe's wife, take a photo of the ideal space. Post it right above that space.
2. Write down the steps, give a learning kid (or oneself) only ONE step per round.
"Any garbage, throw in garbage can."
"Any personal items, put in personal bedroom (or office)."
"Put all socks together."
"Put all dirty dishes together at sink."
"Put every remaining item in its home."
3. Especially with kids: "I will give you $3 to clean the room." When they say it's all done, say, "Ya? Cool. Thanks :) Now, just in case anything got missed, I'm going to pay myself $1 out of the $3 for anything I find. [Excited:] Do you think I'll find anything?" My kid is always very motivated to newly find all the things that were "already done."
My kid can "see" way more now. It didn't change his personality or his brain -he is still unbothered by stuff being about, but now I need only write "clear room" and everything gets done. He has learned how to see even though he isn't a person who naturally sees. Just like I seem to have learned some math, despite my natural state.
For sensory need, often one large item can take the place of 40 smaller ones. So instead of a towel, five plates, a TV, six small stuffies, two beer cans, and seven books, one body pillow or one dimmable light switch or one weighted blanket can achieve the same sensory effect, with 0-1 thing to put away!