Just as a farmer plants seeds and expects a period of harvest, whatever you sow- you will also reap. Whatever you do unto people will eventually be done unto you. When you choose to treat people right, you will also discover they often reciprocate & behave more positively towards you too. It may take time as seeds often take time to grow, but your words/actions/thoughts eventually always yield their harvest. They may return to you as love, money or happiness.
You should therefore be kind & courteous to people irrespective of circumstances. Remember your actions or even your reactions will comeback to you somehow. As you sow, so shall you reap.