Why Would Germany Concerns About Trump's Decision of the U.S. Withdrawal?

On June 15, U.S. President trump said he would reduce the number of U.S. troops in Germany to 25000. And he accused Germany of making insufficient contribution to NATO's military expenditure sharing. Actually early in this June, the media disclosed that the United States was about to withdraw its troops from Germany, which aroused widespread concern in the international community. Observers generally speculate that Trump "declined" the invitation to German Chancellor Angela Merkel

The retaliation for not going to the United States to attend the group of seven (G7) summit may also be the reaction of the United States to Germany, NATO's second-largest economy, whose military spending has not reached the standard of 2% of gross domestic product (GDP) for a long time.