Picture yourself meeting someone (girl or guy) for the very first time. They are overly optimistic (in your words). How would you respond to them? How comfortable will you be with them? And how much will you enjoy the time you spent with them?
If you respond positively to it, other people will generally respond positively to it too.
However, when you talk about being optimistic when a person doesn't feel that way, it's kind of a sad situation to be talking about. People like this will probably be derpressed. And when other people actually find out how shytty the person's life is, they'll get more pity instead of care. Better to fix the pessimistic state of mind first instead of put up an act.