Some YouTuber's theory is (xD) that he finally met a trainer that didn't talk either.
Think like this: You are always forced to battle someone when you came into someone's view, you don't have a choice.
This is the same for Red, who experienced this in (Fire)Red, (Leaf)Green and Blue - and the same for your character in (Heart)Gold, (Soul)Silver and Crystal. :3
Otherwise, there isn't really a reason. I guess otherwise he's just surprised to see someone at the top of Mt. Silver.
Hope I helped. :)