This is the result of the High German Consonant Shift [1]. English - many accents, at least - retain the dental fricatives (th-sounds) that were shifted to alveolar stops (d and t sounds) in High German. Dental fricatives are relatively rare sounds among the world's languages. Icelandic and European Spanish are the only languages I can think of that have dental fricative phonemes.

[1]: http://en.wikipedia.org/wiki/High_German_consonant_shift