Some people already touched on these, but this is basically my considerations:
People tend to give up too easily or early - a few kills behind won't mean we stay behind - I've won games where we have been down on kills the entire game
People like to surrender when the other team is already in the base - game won't last long either way, and I like letting the other team finish it (or chose not to, which we can leverage to a turnaround - seen it happen)
Flamers and trolls - if you don't want to carry your weight and play a team game (or flame and be a bad sport because someone else is doing poorly) I have no respect for you and will not give you an easy way out.
If none of these three conditions apply, I will consider surrendering. But first, I consider the following:
Do we have a chance at winning a team fight if we play a bit smarter - if yes, communicate strategy to team and attempt team fight. If fight is lost, consider surrender
Can we hold out until we are strong enough to face them in a fight? Turtling works to an extent (especially if we have a late game carry)
At some point, the enemy team will stop getting more ahead, and we can catch up (can't do much more than be 18 and have full build) - if we can survive long enough to get there, does our team comp have a chance at contention
Backdooring is still a thing - can we successfully backdoor an inhib or two to help?
As you can see, there are a lot of considerations before I agree a game is well and truly lost with no contest. I try to keep things in the reasonable doubt range - not holding out for a long shot.
Also - Never surrender, and never give up (because Galaxy Quest is awesome)