I think we should learn subjects that we like instead of learning something that is useful.
First,If we learn subjects that we like,we will not regret of learning this even when we receive bad results.But if we choose one subject to learn only for its usefulness,we may lapse into depression and even doubt our intelligence and ability to learn if we fail to achieve any progress.
Second,subjects that we prefer can easily be transformed into useful one if we put enough efforts,while it might be harder vice visa.
To define whether a subject is useful or not depend on how much money we can get from the job where relevant knowledge can be applied.When we invest our time and energy constantly into the subject we like,it's possible to get high salary that we want.However,it may take your whole life to search for interest from useful subject that you are forced to learn at the first place.
Third,it's common to find that we learn faster and better than our peers in the subject we like,so it brings a great sense of achievement and in turn,it motivates you to learn further.On the contrary,useful subjects can not serve this function if you dislike it ,so you are more likely to give it up.
on the summary,I prefer learning subject we like than subject that is useful to us.