Duty and responsibility, moral dilemmas and temptations, relationship friction, losing an advantage, no-win scenarios, dangers and threats

There are many unfortunate times in life where a character must make a choice that results in leaving someone behind. The fallout in this scenario will depend on many factors. This entry will focus on a situation in which the character doing the leaving has no choice in the matter. 

Being in a life-or-death situation and having to leave someone who is injured 
Escaping from a cult or kidnapper and leaving other victims behind
Escaping a dangerous homeland alone because there isn't enough money to take the whole family
Fleeing a natural disaster and leaving a friend behind because they were unwilling to leave
Facing an impossible decision that requires choosing between loved ones (Sophie's Choice)
Leaving a dangerous area on the last transport out that doesn't have space for everyone
Running away from an abusive home without taking younger siblings or children
An injured ally refusing to go on (because they're a detriment), forcing the character to leave them
Retreating soldiers having to leave their wounded behind
Being conscripted into military service and leaving a spouse and children to go to war

Inconveniences arising from not being able to do what the person left behind was responsible for: starting a fire, navigating the stars, hunting, bartering, talking their way out of trouble, etc.
Missing the one who was left behind
A loss of the person's financial assets
No longer being able to use the person's network to get advice, financial support, intel, etc.
Having no one to keep watch, share the workload, help forage, etc.
The character having to change their plans because the abandoned person is no longer there
No longer being able to go to that person for advice or to strategize
Difficulty sleeping or eating