A phrase expressing that one can't help doing something; formed with the negative (ない) form + ではいられない (roughly equivalent to 'can't help but...').
彼女の悲しそうな顔を見ると、何も言わないではいられない。
When I see her sad face, I can't keep myself from saying anything (I have to say something).
そのニュースを聞いて、すぐに電話しないではいられなかった。
After hearing the news, I couldn't help but call right away.
おもしろすぎて、笑わないではいられないよ。
It's so funny I can't help laughing.