As a mercenary, MacCready likes it when you ask people for more money for quests, and criminal activity like picking owned locks. Due to his backstory, which we're not going to spoil for you here, he also likes you helping out children and parents, and throwing in a bit of sarcasm here and there also helps. He doesn't like you being too generous or trying to persuade others by making an appeal to their better nature - and, to be fair to him, at this point he probably doesn't think people have one