Every werewolf needs an anchor. It's something taught to children while they're learning please and thank you. An anchor needs to be pure. It can be a memory, an object, an emotion, a person. The best anchor is something simple, a constant motivator. This anchor keeps you grounded, keeps you strong.
There are a lot of complicated matters in Derek's life. Being an Alpha, a roll he was never supposed to take on, is one of them. He feels both older than his age and utterly unprepared. His relationship with his Uncle Peter, trying to get his condemned house restored, his alliance with Deaton. Derek tries his hardest. Sometimes he just doesn't know how to handle what's thrown at him.
If it's not all that complicated, it's difficult. Eight years and Derek is still in mourning. He needs to be reminded every day he's not at fault. It's getting harder to remember his parents' faces, Laura's laugh. His pack now consists of hormonal high school students. He's learned how to deal with these things, but that doesn't make it easy.
The only simple thing in his life is a hyperactive teenager with a penchant for dog puns. Stiles was the first person Derek trusted after returning to Beacon Hills. When Stiles had every reason to run, he stayed. Derek can't help but return such loyalty. There is no uncertainty, no fear, in his relationship with Stiles. Love can be simple like that.