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Neither Bob nor Alice will willingly be together for fear of harming the other in some way.
Bob and Alice can't be together because their families get along too well.
Bob and Alice don't like each other but their families want them to be together.
Turns out that the external objections to their relationship are just an excuse, and either Alice or Bob doesn't really want to be with the other and/or are driven only by lust or money.
Bob and Alice's families dislike the idea of them being together, but it turns out the reason is they thought it was a trap by the other family...
Double Subverted: ...but they still don't want them together.
Parodied: Bob and Alice's relationship is not really a secret, and it is blatantly obvious that their families aren't opposed to them being together, they might even outright approve of their relationship. But Bob and Alice still keep sneaking off to meet in "secret" and constantly have long melodramatic dialogues about how no one will ever understand their love.
Zig Zagged: ???
Averted: Neither Bob's family or Alice's family minds them being together.
Lampshaded: "Is it really that big a deal if Alice and Bob are together? I mean, I know their families have been fighting for years, but come on!"
Defied: "Look, the last thing I'd want is for my son to be with your daughter, but as long as they're happy, then let's let them be." (And Alice's family agrees)