ものだGeneral truths and proper behavior
ものだ presents something as a natural tendency, a common expectation, or the proper way to behave. It often expresses the speaker's view of what normally happens or what people generally should do.
From one case to a general pattern
This construction does more than report one event. It presents the statement as something characteristic or ordinarily expected across many cases. The generalization may be widely shared, but it still comes from the speaker's view of how the world normally works.
Natural tendencies
Shared norms and advice
When the statement concerns behavior, the same generalizing force becomes advice or admonition: this is what people properly do. The negative form presents conduct as something people generally should not do. Rather than giving a direct order, the speaker appeals to a shared norm, which can sound firm or moralizing depending on the context.
What one should or should not do
General pattern versus a particular expectation
ものだ describes what generally happens or what people properly do. はずだ instead expresses an expectation about a particular case based on reasons, plans, or known information.
More examples
経験を重ねると、物事の見え方も変わるものだ。
As people gain experience, the way they see things naturally changes.
Analyzeどんな仕事にも、慣れるまで時間がかかるものだ。
Any job takes time to get used to.
Analyze親しい間柄でも、礼儀は必要なものだ。
Courtesy is necessary even in close relationships.
Analyze判断に迷ったときは、基本に戻るものだ。
When in doubt, one should return to the basics.
Analyze感情に任せて大事な決断をするものではない。
One should not make an important decision on impulse.
Analyze計画はいつも予定どおりに進むものではない。
Plans do not always proceed according to schedule.
Analyze