An expression showing a strong feeling or impression (e.g., 'I am extremely.../I can't help but feel...'); often follows nouns or na-adjectives with で.
被災地のニュースを見ると胸が痛くてならない。
Watching the news about the disaster area makes my heart ache deeply.
チームが優勝したと聞いて、嬉しくてならない。
Hearing that the team won makes me extremely happy.
彼のことが心配でならない。
I am terribly worried about him.