There are many things that cause toenails to thicken.
For a start, thickened toenails are mostly seen on mature adults! Some just think thickening happens with age, but I believe that age and continual wearing of shoes over the years, and rubbing against the big toenails go hand in hand and I believe this trauma leads to the thickening of the large toenails particularly.
The other major cause of thickening toenails, is fungal infestation ( or onychmycosis) and if this is the cause then the toenails are often also discoloured, yellow/brown looking. They can also be quite smelly as well.
Whichever the cause, and there are other causes as well such as trauma injury etc. it is not up to us to either diagnose or to treat this condition ... It is a job for the podiatrist.
The good news is that a qualified person can reduce the thickness by filing and relieve the pain that rubbing against the inside of the shoes causes.