Everybody learns things from their childhood, be it their good manners, bad manners, or something as small as speaking manners.
And just like this They learn things in their childhood like playing, seeing some truths, some lie, stealing and many other things.
All it comes downs to environment and how the child is getting raised most of the time.
So, what forced Harry to learn stealing did it something happen when he was a child or something else entirely.
