判断・推量N3
〜ものだ
it's natural that... / one should... / how...!
Structure
V/い形（普通形）+ ものだ
Explanation
Several uses: ① general truth; ② recollection or exclamation; ③ obligation. Attaches to plain forms + ものだ.
Examples
人間は間違いを犯すものだ。
People are bound to make mistakes.
子供の頃は、よく川で遊んだものだ。
When I was a child, I used to play in the river.
Note
ものだ expresses general truths, exclamations or recollection; note the difference from もの (reason).