It's a collocation -- words that occur together simply because "that's the way it is" (although, in some cases, there is a logic).
I think that they live here.
Yo creo que ellos viven aquí. (lit.: I believe ...)
She is thinking about it. Ella piensa en ello (lit.: She thinks "in" it).
He calls his mother. Él llama a su madre (lit.: He calls "to" his mother) (Here, there is a logic to the collocation, use of the "personal a").