she didn't even thank Littlefoot for saving her life. That's not like Ducky's character so I conclude that this has happened many times.
Based off Ducky's well known character of gratitude, the fact that she didn't thank Littlefoot for something makes me think, and here's what I think (Ruby). From personal experience, I have a pretty good theory. I have always been the type to thank everyone for everything. But something happened. Certain things others did for me happened so often, my brain registered them as routine instead acts of kindness and made it more difficult to remember a "thank you." For example, my father would always get the milk out for me in the morning when I got up, and I would say "thank you", or the more casual, "thanks" when I was still tired. But after it had happened for so long I would sometimes forget to say "thank you." I still appreciated it, but it just registered in me that this was "supposed to happen." I still thanked him for things he did less often, but in a routine situation, it became tough. Because of this, I believe that yes, this probably happened to Ducky so often that it became "part of her life", not as much "someone helping her" as it used to be. Realizing an act of kindness made me remember my manners. When routine rubs out realization, the mind sometimes overlooks the gratitiude that it should express.