Is it good practice to set all database columns as NOT NULL?

I'm a newbie and my answer may be totally asinine, but here's my personal take on the subject.

In my humble opinion, I don't see the problem with allowing ALL fields except primary/foreign keys to be nullable. I know many of you cringed as soon as I said that, and I'm sure I heard someone cry out, "Heretic! Burn him at the stake!" But here's my reasoning:

Is it really the job of the database to enforce rules about what values should and should not be permitted - except of course as needed to enforce things like referential integrity and to control storage consumption (by having things like max chars set)? Wouldn't it be easier and better to enforce all "null vs. not null" rules at the code level prior to storing the values in the database?

After all, it's the job of the code to validate all values prior to them being stored in the database anyway, right? So why should the database try to usurp the code's authority by also setting up rules about what values are valid? (In a way, using not null constraints except where absolutely necessary almost feels like a violation of the idea of "separation of concerns.") Furthermore, any time a constraint is enforced at the database level, it must necessarily be enforced at the code level also to prevent the code from "blowing up." So why do twice as much work?

At least for me, it seems like things work out better when my database is allowed to simply be a "dumb data storage container" because inevitably in the past when I've tried to use "NOT NULL" to enforce a business rule which made sense to me at the time, I end up wishing I hadn't and end up going back and removing the constraint.

Like I said, I realize I'm a newbie and if there's something I'm overlooking, let me know - and try not to butcher me up too bad :) Thanks.

It depends (on the datatype)

Think about this, If the immediate technology that interacts with database is Python I shall make everything NOT NULL with a proper DEFAULT.

However the above makes sense if the column is VARCHAR with default as empty string.

What about NUMERIC, It is hard to come up with default values where NULL can convey more details other than simply set to DEFAULT=0

For BOOLEAN still NULL makes some sense, and so on.

Similar argument can be carried out for various datatypes like spatial data types.