An individual human existence should be like a river--small at first, narrowly contained within its banks, and rushing passionately past boulders and over waterfalls
个人的存在应该像一条河流，开始很小，被紧紧地夹在两岸中间，接着热情奔放地冲过巨石，飞下瀑布

这不就是最普通最常见的“应当”含义吗

委婉的劝告嘛，虚拟语气本身有这个用法