Outlook 2003

My outlook has the email message list panel and below it there is the
message display panel.I click on the email message in the list, the
line gets highlighted blue automatically, and it opens as normal in
the display panel below.

However, if I click on any text in the display panel, the message is
no longer highlighted up in the list of emails, so I lose my place. 
This strange behavior has only started happening in the last few days;
previously, the line would stay highlighted as I clicked on any text
in the display panel.

This makes my work confusing so a fix would be greatly appreciated.

Thanks,
Dan