There is a good reason to set realistic and achievable goals. You are limiting the likelihood of failure and the associated negative emotions while improving the likelihood of success and the associated positive emotions.

On the other hand, setting goals with a good probability of success limits the "amount" of positive emotions not only in the event of success but also on the whole way towards the goal. A more exciting goal equals more motivation.

In the expected utility equation we've got:

A) Expected emotional consequence of failure

B) Expected emotional consequence of success

C) Expected emotional consequence from motivation

A goals person, has a relatively strong A) and therefore sacrifices to some degree B) and C) in an attempt to make A) unlikely. This means setting a goal that is realistic and achievable.

A dreams person on the other hand has a relatively weak A) and therefore maximises B) and particularly C) by setting a dream. This dream would be absolutely thrilling to achieve but unlikely.

Ultimately, it all depends on your individual A).

That's why I am a dreams person.