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Completely, absolutely and categorically logical.
Spock's morning routine consisted of a few, accurately calculated steps; setting off his hypothetical and biological alarm clock, delighting his sleeping crewmates with a soothing Vulcan melody, continuing it through the cafeteria where his Plomeek soup was kindly waiting for him and, the cherry on top —which some Vulcans could argue as illogical and absurd—, he called it a morning in the exact moment he shared a warm handshake with his captain, James T. Kirk.
Cordially greeting your superior is not only logical but strictly mandatory in terms of discipline. Maybe that's what Spock told himself. This justification was automatically dismissed when you understood what that handshake said about them beyond social norms and learned etiquette in the Starfleet academy.
It is said that a proper handshake should last 1.5 seconds tops, alongside a bit of eye contact that doesn't mean anything past cordiality. Thus, that makes two rules both Spock and Kirk consistently broke.
And what made it truly illogical was the way Spock had to make an extraordinary, superhuman effort to hold back from smiling.