What Am I?

I can be built.

I can be torn down.

I can change my appearance,

but that is just one of my many tricks.

Never moving, but sometimes changing,

I can protect you.

I can keep you safe.

However, if you rely on me too much,

you will become isolated, and alone. 

I see you cry,

I hear you laugh,

when you are mad, boy,

you sure like to throw things at me.

I see everything you do.

I hear everything you say.

Yet, I keep all your secrets to myself.

What am I?