You have been gone for months, busy with your quest to save the world and to defeat Alduin. Vilkas understands you have very important matters to tend to, but he's angry with how long you were gone without saying anything. He knows he doesn't have a right to be angry, it isn't logical, but his heart feels betrayed and he doesn't know why. Maybe it's because you never sent a letter to prove that you were still alive or because when you were in town you never visited...He was worried for you.