It's called nostalgia.

"The best thing, though, in that museum was that everything always stayed right where it was. Nobody'd move. You could go there a hundred thousand times, and that Eskimo would still be just finished catching those two fish, the birds would still be on their way south, the deers would still be drinking out of that water hole, with their pretty antlers and their pretty, skinny legs, and that squaw with the naked bosom would still be weaving that same blanket. Nobody'd be different. The only thing that'd be different would be you. Not that you'd be so much older or anything. It wouldn't be that, exactly. You'd just be different, that's all. You'd have on an overcoat this time. Or the kid that was your partner in line the last time had got scarlet fever and you'd have a new partner. Or maybe you'd have a substitue taking the class, instead of Miss Aigletinger. Or you'd heard your mother and father having a terrific fight in the bathroom, Or you'd just passed by one of those puddles in the street with the gasoline rainbows in them. I mean, you'd be different in some way - I can't explain what I mean. And even if I could, I'm not sure I'd feel like it."

Everything in the musuem stays preserved in its glass cases - the Eskimos will always be frozen into the happiness of catching a fish, the sqaw's joy of weaving a blanket will be perfectly captured forever. The birds will stay mid-flight, and the deers will remain as beautiful and skinny as ever. To Holden, this preservation of life equals happiness. To him, it would be great to stay trapped in the moments of youth - where bad things didn't happen and life wasn't a game - where it was only fun.

But Holden's life can't stay frozen forever; his preservation of childhood just doesn't work. His brother is dead, he's been thrown out of school again, and now he's on his own in New York. Preservation doesn't work with life - it only makes you nostalgic.

Nostalgia - longing for something past. When he visits the musuem, Holden experiences terrible cases of nostalgia. He longs for the days when he visited the museum as a child; back before things were different; back before he realized that he was different. And what does he mean by being "different" exactly? He means by growing up. Every time he comes back to the museum, something, no matter how small, has changed. The little things, such as seeing a gasoline rainbow, change you as much as the big things, like when your parents fight. With each change, as he becomes more and more different, Holden becomes more and more nostalgic; for his childhood, his past, and for the safe preserved glass cases of the musuem.

All this longing for the past?
All this longing to be who he was before?
All this longing to not have to grow up?
It's called nostalgia.

P.S. This poem really reminds me of this passage of the book. It's from The Perks of Being a Wallflower (no surprise there, Jordan), and it's called "A Person, A Paper, A Promise."

Once on a yellow piece of paper with green lines
he wrote a poem
And he called it "Chops"
because that was the name of his dog
And that's what it was all about
And his teacher gave him an A
and a gold star
And his mother hung it on the kitchen door
and read it to his aunts
That was the year Father Tracy
took all the kids to the zoo
And he let them sing on the bus
And his little sister was born
with tiny toenails and no hair
And his mother and father kissed a lot
And the girl around the corner sent him a
Valentine signed with a row of X's
and he had to ask his father what the X's meant
And his father always tucked him in bed at night
And was always there to do it.

Once on a piece of white paper with blue lines he wrote a poem
And he called it "Autumn"because that was the name of the season
And that's what it was all about
And his teacher gave him an Aand asked him to write more clearly
And his mother never hung it on the kitchen door
because of its new paint
And the kids told him
that Father Tracy smoked cigars
And left butts on the pews
And sometimes they would burn
was the year his sister got glasses
with thick lenses and black frames
And the girl around the corner laughed
when he asked her to go see Santa Claus
And the kids told him why
his mother and father kissed a lot
And his father never tucked him in bed at night
And his father got mad
when he cried for him to do it.

Once on a paper torn from his notebook
he wrote a poem
And he called it "Innocence: A Question"
because that was the question about his girl
And that's what it was all about
And his professor gave him an A
and a strange steady look
And his mother never hung it on the kitchen door
because he never showed her
That was the year that Father Tracy died
And he forgot how the end
of the Apostle's Creed went
And he caught his sister
making out on the back porch
And his mother and father never kissed
or even talked
And the girl around the corner
wore too much makeup
That made him cough when he kissed her
but he kissed her anyway
because that was the thing to do
And at three A.M. he tucked himself into bed
father snoring soundly

That's why on the back of a brown paper bag
he tried another poem
And he called it "Absolutely Nothing"
Because that's what it was really all about
And he gave himself an A
and a slash on each damned wrist
And he hung it on the bathroom door
because this time he didn't think
he could reach the kitchen.