I don't know, it seems like a natural progression.
Sully is older, a little wiser, probably isn't too keen on seeing his protege, who seems to have something of a deathwish, throw his life away on yet another scheme. It's been a while since I played 3, but I'm vaguely reminiscent of the idea that Sully had a hint of regret sometimes about bringing Nate into this world (although he's glad Nate was in his life), and by the end of it he wanted Nate to settle down with Elena. He doesn't want Nate to repeat his mistakes. Meanwhile, I think Marlowe was right in UC3 when she said that inside Nate is still the same runaway, looking for glory to validate his existence. And not really being as mindful of what it takes to get that glory, as he should.