Stefan: His journal(s) because he only likes sharing his thoughts with you and only you. And he likes you to know everything that happen in his life.
Damon: He only lets you drive his car because he only trust you with his car. He doesn't let anyone else in the car.
Jeremy: His room because he doesn't like people in his room but since he loves you he only lets you in. And you could what ever you want in his room because he trust you.
Klaus: He only lets you use his paint and brushes because he knows that you love to paint anything when you find it interesting.
Elijah: He only lets you touch his hair because it feels nice when you comb your hands through it. And he likes it when you play with it and pull on it.
Kol: He lets you use his bats because he loves you and he finds it sexy and a turn on when you are pissed and start beating someone with it.