Things Derek is not: sunny, talkative, open, smiley, trusting and friendly.
He used to be all that but then his true talent emerged: making the wrong decisions. He got his first girlfriend killed at the age of 15. At 19 he fell in love with a werewolf hunter who burned down his home with most of his family still inside. Last year his uncle (who used to be in a coma) killed his only other remaining family, his older sister and alpha, Laura. He found her (or parts of her) cut in half by hunters. He had to kill his psychotic, mass murdering uncle. That's how he became an alpha. He turned three "self-esteem deprived adolescents" (to quote the psychotic uncle who at that point had resurrected himself). One was killed in captivity, another died by his hands (because an enemy pack used his body as a weapon) and the last one left him for another pack. And then there was that sweet, slightly dorky English teacher he liked who turned out to be a mass murdering dark druid.
You try and be sunny after all that.
There are those who'd say that Derek isn't good at anything, not even being a werewolf (especially being a werewolf).
While the physical aspects of being a werewolf come naturally to him, he is lacking in knowledge about culture, lore and history of his people. He didn't care to learn growing up and when he found himself alone with only his older sister left it was too late. His short stint as an alpha proved that he is seriously lacking in leadership skills.
As a born werewolf, Derek has learned to control his wolf from an early age. To him the wolf is not an entity in itself, it is a part of who he is. He cannot imagine being anything else.