I consider myself a fairly experienced player (thousands of games, well over 600 ranked games over various seasons) and I know what's possible and what's not.
A lot of people who get depressed don't realize several key factors:
If they have one (and only one) fed person - you always have a chance. Getting a pick on that person or forcing a fight when they are not there is almost a guarenteed comeback. You should be looking to make your plays this way.
If they have an early game comp and are winning early game - DUH! This gets a lot of people - So Cait beat your Vayne 3-0 in lane. Yeah....that's kinda how it goes. You should not give up because of that.
If you have a really late game comp - never give up. If you have something like Nasus, Kat, Vayne, Thresh and Yi - don't surrender. Yi can take a tower in about 10 seconds (or less) late game, so split pushing is always viable. With the resets on Yi and Kat, any teamfight where you burst one person down (cough, Nasus Q) can easily be turned. And Nasus, Vayne and Thresh are gods of late game with insane damage and utility.
If you're close to full build. Press tab when that surrender vote pops up. Do your carries already have 4.5-5 items? Play it out for 10 more minutes. Let them get full build. No gold advantage matters at full build.
You have more objectives than you realize. If you have 3 dragons to their 1 - Fuck no, you don't surrender. 12 minutes, 2 more dragons and you're team is going to be buffed to fuck. Even if they get Baron - you can wait it out - the first 4 dragon bonuses are permanent.
All that said - there are cases where it's totally appropriate to surrender.
All lanes lost (so multiple people on the enemy team are fed) - If they have 3 people who are like 5-0 - it's pretty much game over. Even if you kill one, the other two can still do enough to 4v5 your team.
You lose early on an early game comp. If your Cait is losing 3-0 to Vayne - you're in a bad position.
You have no objectives. 0 dragons, 0-1 towers, 0 barons, can't even farm your own jungle anymore - you just aren't going to get gold fast enough to come back.
As their team approaches full build you aren't even half way there. If they have a 6 item Kat and your top and mid laners have 2 items - yeahhhhh - it's probably game.
so, tl;dr - Comebacks are actually really easy if you know how to play in your particular situation. I'd say I see throws/comebacks about 1 in 5 games. There are games that are definitely lost though and it's okay to surrender those and move on.
"No gold advantage matters at full build." Wise words right there, son.