Any reason they should?
Other than that America provides for their defense and rebuilt them from ashes and bailed them out in two world wars and then protected them from communist dictatorship and that America is a western democracy which actually believes in the rights of man and that the US Navy keeps the sea lanes open and that the US has ensured the free flow of oil out of the Middle East? Other than that, no particular reason.
Trump does not believe that Germany has kept up their part of the bargain. Why should we go to their defense under those conditions? We may have ones, but now?