expression, i-adjective
natural; understandable; no wonder
Used to express that a situation or reaction is only natural given the circumstances. Often appears as 無理もない or 無理はない. The phrase literally means 'not unreasonable'.
See also: 無理はない
彼が怒るのも無理ないよ。
It's no wonder he's angry.
初めての海外旅行で緊張する
It's natural to be nervous on your first trip abroad.
当然 is a more formal and objective 'natural' or 'of course', while 無理ない is a softer, empathetic expression often used to show understanding of someone's feelings or actions.
もっとも (尤も) means 'reasonable' or 'understandable' and can be used similarly, but it often carries a nuance of conceding a point in an argument, whereas 無理ない focuses on emotional empathy.
From 無理 (unreasonable) + ない (not), literally 'not unreasonable'. The phrase grammaticalized into an i-adjective-like expression meaning 'natural' or 'understandable'.