I understand that this comment is made in good faith and with the best of intents, but I loathe getting this kind of advice. I'm not trying to be a dick, I just want to let you know that this advice is extremely frustrating for a lot of people suffering from back pain. ESPECIALLY "you look young and healthy"...Yea thanks, I'll probably still look young when I'm in a wheelchair.
PS If your back hurts and your doctor doesn't seem to believe you or care, get a new doctor.
I recently went to one session with a PT to diagnose it, since it's returned again this year. It's probably chronically strained quadratus lumborum or some other muscle involved in spinal extension. Avoiding upright sitting as much as possible definitely helps. I've recently moved closer to things so that I can bicycle to most things instead of driving, and I think that's helped too.
The PT thinks the solution is to train my movement patterns to use more core / glute stabilization rather than dumping the responsibility on the back muscles. There are various exercises to help train those patterns and strengthen then muscles involved.
Personally I find a standing position healthiest: for women in particular, it avoids particular long-term hip flexor problems.