As time pass by you begin to feel like you are no longer on the same page as others.
So you start building walls around you, only allowing a selected few to pass through. You can protect yourself from everyone else who disagrees with you.
Or maybe, all these while, the walls were already built by them. It takes time for you to realise that the reason why you never bump into them is because you keep bumping into their wall.
Or maybe, there is no such thing as walls. Only time can tell.