It's the Booth Buddies situation without the booth. Star isn't unhappy with Tom, more like she's okay with where it is. Marco wanted some affection and he found it with Kelly, who needed it too, hence the "break up buddies". They should be happy with the current relationships status, but they aren't, which can very easily lead to the "we should be happy, why aren't we happy?" dilemma. And judging by the synopsis for Knight Shift they're gonna be forced for the third time to confront a problem regarding their relationship and future.