Originally Posted by
noimagination
My wife has this trouble. IMO, it's because she learned to ride as an adult.
When you're young, you experiment a lot. If you're riding a bike (my siblings and I practically lived on our bikes from ages 5 to about 11 or 12), then you experiment on a bike. Stand, skid, jump, ride no-hands, bash into stuff, and so on and so forth. You try stuff and figure it out, and have a hell of a time doing it.
Adults, on the other hand, get used to knowing what they're doing. When they try something new, they are not as open to experimenting, failing, practicing, failing some more, etc. Whether it is riding a bike, or skiing, or any other activity, most adults get to a point where they are reasonably competent at doing the basics, and that's it. Most adults don't find it fun to try stupid stuff to see what works.
As far as standing while riding: your balance is constantly shifting. The balance between pressure on your right foot, left foot, right hand, left hand, fore and aft balance, is always in flux. Your head is constantly moving around, which, if you're not used to it, can throw off your balance. You have to get used to feeling unstable and to controlling your body in three (four? time?) dimensions. It takes practice. If you're not willing to put in hours of practice, and look utterly stupid while doing so, you're not going to learn.