Well, it is a sad fact that it actually is! It is the pull of gravity! Your body is constantly reacting to the pull of gravity. It isn't something that we normally think about but gravity is always present. Your body is always in a balancing act which other people see as your posture.
When you are talking about posture, it quickly becomes a question of "good posture". With a good posture, there should be a minimal amount of muscle contraction being used to stay upright. To put it in technical terms - the body strives for conservation of energy.
So what is good posture, where is the balance? I'm not sure I know. (There seem be quite a few opinions on this matter.) What I have observed is that when we try to correct our posture, we tend to do it from the the top down. Yet we contact the ground with our feet!
If you think of posture from the head down, then you happily accept the advice to "pull" your shoulders back. (Generally this advice leads to frustration as your shoulder/back muscles tire from all this effort!)
Wait a minute. If posture is a balancing act, then the fact that you are being advised to pull your shoulders back is suggesting that your shoulders are actually being pulled forward! So why not stretch the muscles on the front of the chest so the shoulders can now balance on the body. Having the shoulders balanced is certainly easier than constantly pulling them back!
Of course, the shoulders won't balance easy on the body if the back and pelvis aren't balanced and if we look further down, what are our feet doing? (I didn't say posture was easy!)
And we haven't even mentioned the position of the neck and head! (If the head is forward, then you are making work for your neck muscles.)
So how balanced are you?
Are your muscles having to work too hard against the pull of gravity?
Think of how much more energy you would have, if your body wasn't "working so hard"?!