Hmmmm.... I suspect it's related to physics and your ears.
No, really
Your head (especially at the age where you are likely to try to stick your head between bars without having consumed large amounts of alcohol) is not an even cylinder. The top of your head tends to be wider, and as you go towards the neck, you get narrower. As you try to push through, your ears get in the way and get pushed toward a narrower part of your head.
When you try to get out, your ears again get in the way, only this time, they're pushed toward a wider part of your head, making your head wider. To proceed forward from that point gets rather painful unless something is involved which either makes the opening wide or makes the head narrower.
Now, as an older person (say, 14 or more, depending on the part of the world), putting your head between two bars may often involve... [img]graemlins/1puke.gif[/img]