Boris' words to Balto during the first movie are to try to cheer up Balto and encourage him, in the second movie Boris tries to say what might soothe Balto about Aleu, in the third movie Boris doesn't want to admit that he's afraid of heights. In all three movies of Balto's, every now and again Boris is trying to give a warning.
Boris does his best to comfort Balto when he's a bit lonely on the wooden boat he sleeps on, trying to do things in a similar ways like drinking water and sleeping on his tummy. Boris' own fear is what causes him to fall backwards fast when he tries to tell Steele to leave Balto alone, with growls Balto tries to tell Steele the same thing. Steele or the two bear cubs Muk and Luk it's his irritation that makes Boris shout. But he admits his mistakes and apologizes to Stella as well as explains his feelings to her. His fear of height is what made him do his jokes with Stella first but Boris liked having Stella's company