The lack of sympathy for foreign-born immigrants in this country might have something to do with how few people in the U.S. ever lived in a different country. If you have never had to learn new customs, different cultural values, and another language, you probably don’t realize how complicated it can be.
With the best will in the world, not everyone learns a new language easily. Moreover, many customs are so ‘built in” to the way we interact with others, it can be hard to “unlearn” them. 
Smiling at strangers, for example, is very American. We do it automatically unless we are alarmed or frightened. In some other countries — Israel was one — a woman smiling casually at an unfamiliar man is considered a “come on.” Learning to not smile automatically is difficult. It’s a physical habit learned when we are very young.
I was also surprisingly bad at learning Hebrew. In almost a decade, I never became fluent. My son, on the other hand, was fluent in a few months. He had always been able to out-talk me in English, but a few months in Israel and he could out-talk me in two languages at the same time.
Israel was, overall, welcoming to newcomers. America, not so much. That makes everything harder for someone new to the country. Considering most non-Native people in the U.S. are immigrants or the children or grandchildren of immigrants — being nice shouldn’t be all that difficult. Remember your past. Remember your grandparents. Be civil and perhaps even a bit generous.
If you were never an immigrant, someone in your ancestry probably was. Being kind to those who come here from elsewhere should not be all a trial. Give it a try.
