I don't think he was being dense but had intuition he couldn't place logically. Chapel is saying that T'Pring wants Spock to prioritize her over his career. I think Spock somehow already foresees that even if he does that, T'Pring will still dump him for someone more Vulcan then him. But he can't logically prove it. But it's something he senses, especially with her criticism of his human style quarters.Oh, it was just a playful tap on the head between friends. For all of Spock's intellect, he can be a tad dense sometimes, especially at his currently young age.
And as we all know no matter what Spock does T'Pring will dump him. Spock is being smart, not dense. Guys like this are placed in these impossible situations all the time. Prioritize a relationship over their career and then end up with neither because the other party was just using the career as an excuse to berate the man. I had a neighbor who literally lost a job after such a scenario.