Old English wundrian "be astonished," also "admire; make wonderful, magnify," from the source of wonder (n.). Cognate with Dutch wonderen, Old High German wuntaron, German wundern. Sense of "entertain some doubt or curiosity" is late 13c. Related: Wondered; wondering.
Reflexive use (It wonders me that "I wonder why ...") was common in Middle English and as late as Tindale (1533), and is said to survive in Yorkshire/Lincolnshire. In Pennsylvania German areas it is idiomatic from German das wundert mich.