The reason why you need LiveCycle Designer to edit your form is because it has been created with LiveCycle Designer. When that has happend, it can no longer be edited with Acrobat (because it is no longer a PDF, but an XFA, wrapped in a PDF shell, so that Acrobat/Reader can open it for use. It is as simple as that. XFA is a one way street (some people would say side track or dead-end).
Now, with Acrobat XI, Adobe is no longer throwing LiveCycle Designer after the buyers of Acrobat Pro for Windows. It is now a product of its own. If you have a legal copy of Acrobat X Pro, you should be entitled for a license of LCD; search in the forum here for the according link on the Adobe website.
Hope this can help.