Personally, I believe our allies should help with a couple of things. First, NATO members have a treaty obligation to use 2% of their GDP on defense. Some, including Germany, are not doing so and must live up to that obligation. Germany is only providing 1.19% of their GDP on defensive spending, for example.
Secondly, if the U.S. is providing military assistance to countries by having military bases (e.g. Korea) and other assets in their country, they should help to, at a minimum, defray the cost of that defense.