Using 'summon up' suggests making an emotional effort to recall or imagine something.

Emotional Effort

'Summon up' often requires closing your eyes and picturing a scene or person vividly in your mind.

Intense Visualization

This phrase does not mean calling a person or thing to physically come; it's a mental process.

'Summon up' implies a struggle to find the necessary emotion or quality within oneself.

It is often used when someone needs to overcome apathy or lack of interest for a task.