くせに: criticizing behavior that conflicts with expectations
Use くせに to express dissatisfaction with behavior that clashes with a known fact. Compare the contrast shared with のに, connecting forms, and the force of addressing someone this way.
An expectation from a fact, and the actual behavior
彼は知っているくせに、教えてくれません。 criticizes him for knowing but not telling the speaker. It contrasts the expectation that someone who knows might help with the actual behavior.
The clauses typically contrast states or actions of the same person. 自分で約束したくせに、時間を守りませんでした。 expresses dissatisfaction that someone failed to be punctual despite personally making a promise.
Connecting nouns and adjectives
Nouns take forms such as 専門家のくせに, while na-adjectives take forms such as 暇なくせに. 暇なくせに手伝ってくれません。 blames someone for not helping despite having time on their hands.
An i-adjective connects directly, as in 安いくせに丈夫です. Used about an object, this can express surprise that it is sturdy despite being cheap, so not every use is a direct insult to a person.
Emotion when compared with のに
知っているのに、教えてくれません。 also contrasts knowing with not telling the speaker. Choosing くせに tends to make the reproach stronger, presenting the other person's behavior as unjustified.
A polite ending does not remove the blame. If the purpose is simply to ask someone for their reason, choose a question suited to that purpose, such as 理由を教えてもらえますか。.
Quick check: What judgment appears in 助けてもらったくせに、お礼も言いませんでした。?
Show the answer and why
It criticizes someone for not even saying thank you after receiving help. It conveys dissatisfaction with their behavior rather than neutrally listing two facts.