I'm looking for songs that talk about the difficulty to get to really know the others...?

It can be in any langage. It's for a research I'm doing for a show that we will mount soon. The theme is really that impossibility to be in somebody else's head. Impossible to know what's happening in the other's mind...

the "country" category is a mistake: any kind of music.

"Let Me In" by REM
"Fortress around Your Heart" by Sting
"Nothing 'bout Me" also by Sting

"Don't Laugh at Me", by Mark Wills is an excellent one that fits the description you gave. It's from his 1998 CD, "Wish You Were Here".