You didn't have pain in your lower back when you were a little child.
You most likely had good, balanced posture when you were very young.
When we learned to walk, we held our heads directly over our little bodies and had nice, supportive curves in our lower back.
But, then we went to school and sat on furniture that didn't fit us.
Sometimes we had to sit for hours at a time instead of running around using all of our muscles.
We stopped using all of our muscles.
Or, we started using them in incorrect ways.
Or, both.
Most people use only the same 60 or so muscles on a daily basis.
Since we have over 600 muscles, that means we use only about 10% instead of 100%.
That causes our muscles to be out of balance.
It creates poor posture and back pain.
Or, maybe you continued using most of your muscles, but in ways that caused some to be much stronger than others.
Your posture might look pretty good, but still you feel strain and pain in your back.
That's a clue that your muscles aren't balanced from front to back.
This also causes back pain.
What causes poor posture? There can be many causes, from feet that hurt, to "bad" furniture and car seats, to "bad" habits like slouching.
And, if you hold your head and arms in front of your body for most of the day, like most of us do, the muscles in the front of your body get short.
They pull, or round, your shoulders forward.
Those short muscles pull your head forward and down and make it easier for gravity to pull you even further forward.
When your back muscles get weak, they get overstretched.
Instead of holding you upright, your weak back muscles let your spine get rounded at the top.
This causes you lose the curve in your low back, too.
This posture causes muscle strain in your lower back (and your neck, too,) and muscle strain causes low back pain.