Have you ever stopped to wonder why your students are there? I don’t mean in a deeply existential way, but more, what is their purpose for learning English? I think it can be summarised in two broad categories: generic reasons (business, school curriculum, etc.), and personal motivations (communicate with foreign colleagues to enhance career, the desire to go traveling for a year etc.).
If you are teaching at a school then your students are probably there because it is part of the curriculum. English is one of the many classes that they have to take (of course, they have personal reasons in addition to this).
The variety of uses that the English language is put to has given rise to ESP, English for Specific Purposes. This is an entirely separate branch of TEFL that covers things such as business English, academic English and the English of specific industries. If you are teaching adult classes, they probably have specific goals in mind already. Obviously a business English class has a clear theme, and even conversational classes are aimed at increasing general language skills in an informal setting.
Personal reasons for learning English much more important than the generic reasons mentioned above. Your students must have some interest or desire that is pushing them to learn English. Some of my students told me that English was a very useful language – a language that will enable them to travel anywhere in the world. Others have told me that they want to communicate more easily with their superiors at work in foreign branches. Still others want to live and work abroad.
Knowing the personal motivations that your students have can enable you to tailor your lessons to their needs, and it can help you to keep them enthusiastic. So, find out why your students are there, and keep on pushing them towards their goal.