What is the difference between a dream and an obsession?

These two have meanings that overlap; a dream can be an obsession. In general, if you admire someones intentions you would be more likely to refer to them as a dream, and if you disapprove you would be more likely to refer to them as an obsession. Obsession has some connotation of mental illness, for example, there is a mental illness called obsessive compulsive disorder. Dreams, in comparison, carry some connotation of nobility. We dare to dream great dreams.