Why does my chicken go dry when I boil it?
I may be wrong but I'll give it a shot.

When you heat chicken (or any other muscle for that matter) it tenses up. As it tenses up it is essentially squeezing juices out, because all the fibres are closer together.  Just think what happens when you slice a rare steak without letting it rest, there will be blood everywhere. 

In general I believe boiling anything is bad practice. A slow simmer maximum is required to retain all the desired qualities. Hence the saying "low and slow" which I take quite literally band apply to just about everything culinary wise. From cooking stew to proving bread, the longer the better. 

Edit:

I wrote that then went in the shower and remembered pasta &amp; potatoes, so maybe low and slow unless it's a starchy carbohydrate. Or if you are searing, but often a blazing hot seat is accompanied by a nice rest.