"Why are you studying English?" Together with Shakespeare and Dickens, this is a classic of the English language. I mean, how many of us have asked our students that question? The thing is that we do not start shooting them with this question the moment they sit at their desks and have barely taken their notebooks out of their bags. That would be a bit intimidating rather than relaxing. So, what about disguising our genuine interest and engage them in an activity if we want to find out their reasons for learning English?
We could carry out a survey. That's a typical activity in the classroom, and every teen or adult knows how they are carried out, so they need very few instructions, indeed. We could provide them with a list of reasons for learning the language and they number them in order of importance, and then they can share their results with the rest of the class, so that a pie chart can be drawn for the class, which shows the percentages reached. It provides time for the student to reflect upon their answers; it fosters peer communication when they have to share results; it is a very good way of breaking the ice and an excellent opportunity to get to know each other; and every student is represented in the outcome (the chart).
A much less time-consuming activity could be the teacher-friendly Finish the sentence. We could write some unfinished sentences on the board for them to finish (kind of: I want /must / have to /am learning English because ...), something which can be done orally, and then used as a springboard to link students with the same interests or work with similarities and differences.
Anyway, never get rid of the old straightforward question. It could be useful when there is only one new student in the group, ...but let them sit down and relax first!
Now, since there are different reasons for learning a language, I think the best way to motivate them is to vary the kinds of activities and tasks. The more variety, the more chances we have to address a wider range of students. Besides, that will give us a chance to see which activities they feel more (or less) comfortable with, which will in turn be beneficial to create a good learning atmosphere. Something I usually do is to surprise them with the unexpected. For example, surprise adults by teaching commands using cartoons or films for kids, work on physical descriptions with adolescents watching a video clip from the 90s... I mean, something they are not used to, and something that shakes them out of routine and makes them think "What will come next?" I have found that truly gratifying.