aptitude in interactive mode.
When you are displaying the information of a package in
aptitude's interactive mode, you can press the I key to switch between
Related Dependencies and
Why Installed in the information panel.
You can make this more obvious if you enable displaying tabs in the information panel:
Hit Ctrl+T to activate the menu, navigate to the
Options menu and select
Search for this option:
[ ] Display tabs for the information area
Activate it by pressing Enter or Space when it is selected. It will get checked (
Hit Q to close the options tab.