For me it was when I am roleplaying a real life airline and I'm desperately trying to be the launch customer of a type only to be beaten (sometimes by seconds) by another player, this happened recently with the E170 (Crossair was actually the original launch customer irl, they were to take delivery of the second E170 ever built but the order was cancelled when they re-organized into SWISS)
Other times its when i start a airline with the intent on operating a certain type, enter a bad financial period only to end up never being able to purchase the type by the time i recover cause its gone out of production but this happens once in a blue moon and when a airport i want to really fly into has no more gates
What about you guys, what times have you felt salty