This is only explanation I ever understood. ready.....?
YOU CAN'T SCALE THE AIR DOWN
Not scientific I know, but non-dimensional constructs I find hard to visualise and this helped.
If the sensible and educated responses above seemed a bit abstract they may be a bit more digestible with this in mind. Or not.
Personally I put reynolds no.s in the same category as long division. Don't matter how much effort I put in I just can't do it.