By "Tuck" I believe that the question is referring to Angus Tuck. Chapter 12 contains the answer to this question. It is during this chapter that Angus takes Winnie out onto the pond in order to talk to her about what exactly immortality means for him and his family. It is clear that Angus is not excited about being immortal, and it is also clear that he thinks nobody else should have his gift/curse.
Angus Tuck uses the stream's current as an analogy to explain to Winnie why being immortal is not right or good:
"Life. Moving, growing, changing, never the same two minutes together. This water, you look out at it every morning, and it looks the same, but it ain't. All night long it's been moving, coming in through the stream back there to the west, slipping out through the stream down east here, always quiet, always new, moving on. . . Always coming in new, always growing and changing, and always moving on. That's the way it's supposed to be. That's the way it is."
Life's constant change is important to Angus, and he is no longer a part of that natural life cycle. He explains to Winnie that he is no longer a part of the "wheel," and he does not like it. He admits to Winnie that he would love to once again be a part of that wheel. Winnie does not understand why. She cannot fathom why anybody would want the ability to die. Angus gives a simple and wonderful explanation as to why he really does not like being immortal anymore. In his opinion in order to know that you are living, there has to be the possibility of death. Since he cannot die he does not feel that he is really living. He just is. To Angus his "life" is like that of a rock. It cannot die, because it is not alive. It is just stuck in time:
"Being part of the whole thing, that's the blessing. But it's passing us by, us Tucks. Living's heavy work, but off to one side, the way we are, it's useless, too. It don't make sense. If I knowed how to climb back on the wheel, I'd do it in a minute. You can't have living without dying. So you can't call it living, what we got. We just are, we just be, like rocks beside the road."