What theme is explored in Tuck Everlasting? How is this a theme that we can relate to?

The theme is that when you get what you want, you may not want it when you get it. 

The theme is the message of the story.  A lot of people think they want to live forever.  Dying is frightening, so why wouldn’t it be great to never die?  The Tucks teach us that dying is a part of life, and if you wish to live forever you should be careful what you wish for. 

Pa Tuck uses an example of a rowboat on a river to explain to Winnie what it’s like to live forever. 

"But this rowboat now, it's stuck. If we didn't move it out ourself, it would stay here forever, trying to get loose, but stuck. That's what us Tucks are, Winnie. Stuck so's we can't move on. We ain't part of the wheel no more. Dropped off, Winnie. Left behind. (Ch. 12) 

The Tucks did not ask to be immortal.  When they drank from the spring, they had no idea that would be the result.  However, now they have what so many people want.  They cannot die.  In the end, this is not all that it’s cracked up to be.  The Tucks feel like they cannot participate fully in life, because the world leaves them behind. 

Winnie has a similar experience, albeit on a smaller scale.  She desperately wanted adventure.  She wanted to run away from home and see the world.  She felt stifled at home with her family 

I'm tired of being looked at all the time. I want to be by myself for a change. … I'm not exactly sure what I'd do, you know, but something interesting—something that's all mine. Something that would make some kind of difference in the world. (Ch. 3) 

Winnie got her wish.  She had an adventure.  However, it was not the adventure she was expecting.  After she accidentally saw Jesse Tuck drink from the magic spring, the Tucks kidnapped her.  They wanted to have time to explain.  Winnie got involved in the death of the man in the yellow hat and a prison-break. 

Winnie learned that she is lucky to have a family.  She wished to be on her own, but she saw what that did to Miles and Jesse.  They could never have a real family, because their wives and children would not be immortal.  Winnie realizes that her quiet, family-centered life is actually a luxury.

This is a theme anyone can relate to.  We have all longed for something, and then realized we didn't really want it.  Most of us have also wanted change until we had it, when we wished for things to be the way they were instead.