As others have noted, when playing solo, the standard is set a bit lower. If the endgame is, say, 90 percent certain, and I'm tired of the game and don't feeling like grinding the game out for another turn or two, then I have no guilt about cleaning up and moving on to another game.
When I'm playing someone else, I will rarely call a game unless it's a mathematical impossibility for someone to win. I've seen some ASL games turn around dramatically with a good or bad sequence of rolls.
One of my friends is a bit of a defeatist, and there have been a couple of times when he has given up and conceded too soon. Perhaps the chances of him winning were not looking good, but IMO he gives up too easy. I try to encourage him to hang in there and finish the game, but I'm not going to beg him. I figure that if he's convinced that he can't win, then at that point he's not enjoying the game any more. If I insist of playing, I'm concerned that he may think that I'm piling on and kicking him when he's down.