The problem: your whole life you have been told that you should sit up straight.
Your parents may have started this. Then at school and work they would have carried it on and now you may even tell your children "sit up straight"!
Of course sitting with a nice straight back preserving the "J" curve in your low back and keeping your chest forward is so much better than slouching!
I want to time you how long you are going to last... Maybe 20 minutes? Maybe not...
Be honest, how long and how staright you are during your 8 hour desk job?
Sitting straight puts excessive pressure on the joints of the spine, it loads the discs and weakens the spinal muscles.
Because all of your weight centres on the lowest three joints of the back this makes them uncomfortable and the position impossible to maintain for longer than 20 minutes.
It's for this reason most people slouch at work as sitting straight is not comfortable or easy to maintain.