So, Cassidy is just all messed up in a variety of ways (terminal illness being the worst, alcoholism not the least). Her personality is generally too brash and arrogant to allow for a whole lot of comforting, but her disease causes her to be in pain nearly all the time, and sometimes she can't quite drink it all away. Cillian is really soft-hearted, so he wants to help. Her problems are kind of overwhelming to him, though.

Blah blah blah! Nobody cares xD