Why is sitting with "good posture" tiring/uncomfortable?

Is it so terrible to prefer slouching?
Why shouldn't we just sit with what is most comfortable?
What can really be the effects of such?
Is good posture just something that is seen as "socially" desirable rather then good for the back?
If it was good for humans, then why does it become to tiring?

Actually, sitting with poor posture will eventually cause back pain, spinal dysfunction, joint degeneration, etc.
The pain when we have when we do proper posture is because our body is used to poor posture, so we're working muscles that don't normally get used.