This poem, to me, perfectly encapsulates how emotionally draining it can be to deal with others, sometimes. To be surrounded by the foreign, no matter how much you smile and laugh, does wear on you, crumbling your spirit slowly "like the water drops which...wear the rocks to powder." It comes from the concept that Lowell uses called "imagist" where form and rhyme conform to natural speech rather than poetic rules. There is no meter and no system to the way she creates her imagery. Hence the title, "aliens"...strange objects or in this case, people, who invade our space and create an almost erasing of who we are. The image of being worn down by talk that seemingly means nothing.
Even if we don't take the title, "Aliens" to mean foreign peoples in terms of culture, it can simply be strangers. I think we've all had a day where every interaction takes something out of you. We are all alien to one another on some level, and our inner selves are our sanctum and shield. Our minds and self concept are ones that protect us from being totally withered away...a small piece at a time.
When you're trying to study, finish a project, or just get something done, the little distractions like our phones, friends, family, etc., tend to hinder us from reaching those goals. While we might welcome to distractions at the time, the impending deadlines stress us out and wear us down. The line "Like the water-drops which slowly wear the rocks to powder," is a metaphor for our willpower to focus. While we were once rocks working on our assignments, motivated and powerful, the little distractions wore us down to stressed out former beings of ourselves, trying to succeed with what little bit of power we have left.
Once we have been worn out and actually finish the work, our former weak selves grow strong again, where we can analyze the title of "Aliens." Our non-stressed out selves are aliens to our stressed out selves and vice versa.