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To not express gratitude or thanks
彼は感謝しない態度を取ることが多い。
He often takes an ungrateful attitude.
感謝しないと、相手は傷つくかもしれない。
If you don't thank them, they might get hurt.
This phrase emphasizes the absence of gratitude, which can be seen as rude or insensitive in many social contexts.
In Japan, failing to express gratitude can damage relationships, so this phrase is often used to criticize someone's behavior.
Even if you don't feel grateful, it's often polite to at least acknowledge the other person's effort.
Combination of 感謝 (kansha, gratitude) + しない (shinai, negative form of verb 'to do')
In Japanese culture, expressing gratitude is highly valued, so this phrase often carries a negative connotation when referring to interpersonal relationships.