SQL Server Management Studio - Adding/Moving Columns require drop and re-create?

Why do I get message that the table needs to dropped and re-created when I add/move columns? I believe this happens after adding foreign key constraints.

What can I do to add new columns without dropping table?

If you're more interested in simply getting SSMS to stop nagging, you can uncheck the "Prevent saving changes that require table re-creation" setting in Options->Designers->Table And Database Designers. The table(s) will still be dropped and re-created, but at least SSMS won't pester you quite as much about it.

(This assumes you're working in an dev/test environment or in a production environment where a brief lapse in the existence of the table won't screw anything up)