He is? O_o Hmm, his behavior might be throwing my memory off.
Strongbow's behavior was different when he was young. At the start of Wolfrider, he is headstrong, with an impulsive streak. He follows Bearclaw around like a kid-brother, trying to impress him. I got the impression Bearclaw was a bit like a father figure to him. We see his mother, (Trueflight?), but not his father in these scene... just Bearclaw. Bearclaw is alternately fond of him, tolerating him and reprimanding him.
Remember the "why are you the only one to hunt bear?" incident?
Strongbow doesn't really seem to become sullen until after the death of his daughter. Before that, he seems to have more of a sense of humor. We see Bearclaw and Strongbow teasing each other, while Strongbow teaches Crescent how to shoot a bow. I had the impression, just from those few, brief scenes, that he was very proud of her. He seemed very content at that moment.