The Young and Dawes plans were devised to reduce (and eventually stop) reparation payments, to heavily invest in German industry, and to sell Germany some American machinery.

Rebuilding of Germany started under the Weimar Government, not Hitler's, and the Allies wanted Germany rebuilt in order to provide a good trading partner (people desperately wanted to avoid war after the first world war). A strong Germany would also provide a strong bulwark against Communism from the East, to stop ideology as well as any military advance, by using Germany as a buffer zone.

So by reducing the payments Germany owed, and investing lots and lots of money into German industry, they kick started Germany back into being the economic powerhouse it was by the start of the second world war.

These factors are what contributed to Germany's fast recovery, in addition to its natural wealth in resources, and good trade infrastructure. Railways and large, navigable rivers made it easier to build Germany back up again.

Edit: A longer answer can be found here, in this older AskHistorians thread. Particularly the answer by /u/DuxBelisarius

Germany defaulted on war reparations IIRC in about 1934. You have to also realise that such obligations are only as good as the willingness to enforce them. Germany did suffer for the contempt the Nazis (arguably wisely) showed for international financial stability, but there was a context of a wider trade war going on as nations tried to secure their position during the Depression.

Germany was never in a position to do well in this man many recognised her plight and sympathised with it.

There is good stuff on the economics of Nazi Germany in Adrian Tooze'sThe Wages of Destruction. One revelation is that Nazi Germany was in reality no superpower, but was running on the absolute limit of banana republic economics right up to the war. But the image was certainly projected with great success.