FROM THE START IT HAS BEEN A GAME FOR US.
Masky is obsessed with you because you are obsessed with him. This isn't something to be happy with though. Masky is watching you, so he can make sure that you don't find out anything about Slender, the others, or himself. He has found out almost everything about you, and decided that you were a threat. Since you are a threat he had decided he was either going to kidnap you or kill you, depending on what Slender decided. For now he's just watching you...Waiting for the order to make his move.