Quoting from ops manual part B (QRH) preamble for the B757.
" There are some situations wich always require declaration of an emergency ....etc"
it lists the following
" Engine failure or Fire
Wheel Well Fire
Cabin smoke or fire which cannot be immediately eliminated or extinguished
One source of AC power remains
One Hydraulic system remains
Checklist contains the phrase 'declare an emergency' "
and obviously
" any other situation crew determined ...... may have adverse effect on safety of flight."