This is a really good point. There was a point in my life where if I extended some form of kindness or friendliness to a person, either a total stranger or someone I just didn't know very well, and they didn't return the friendliness, I would take it very personally and feel annoyed. This is completely not the case for me now. If they do return the friendliness, then I feel grateful for it. If they do not, then I take it as an indication that maybe they didn't see/hear me, misunderstood, are caught up in their own dukkha for the moment, feel awkward, etc. Rather than anger, there is understanding and compassion. No matter what the case, I am filled with warmth and understanding. I even enjoy going about my day doing mini-mettas toward people I pass, a simple "May you be happy and well."
“This is the true joy in life, the being used for a purpose recognized by yourself as a mighty one; the being a force of nature instead of a feverish, selfish little clod of ailments and grievances complaining that the world will not devote itself to making you happy…I am of the opinion that my life belongs to the whole community and as long as I live, it is my privilege to do for it whatever I can.”
--George Bernard Shaw, Man and superman
Here's another good one on the topic that I just happened to come across:Metta: The Philosophy and Practice of Universal Love
by Acharya Buddharakkhitahttp://www.accesstoinsight.org/lib/auth ... el365.html