Why I keep being asked why I'm not smiling?

People ask me why I'm not smiling, constantly.Actually no not "people",only men! Women NEVER ask me that question! Why should I smile when there is nothing to smile at? Yes I have problems,serious problems like family,job,unemployment,school,non-exist… dating life. I'm not a happy person but I don't go out to show my unhappiness to people,I try to look pleasant but they want me to f***ing SMILE.Why is that? Why don't people ever ask MEN to smile? Why do women have to smile? Do we,as women have to be pleasant and easy going no matter what the situation is? Or do men underestimate our problems? When you tell me to f***ing smile THAT WILL NOT MAKE ME SMILE.If you want to make me smile you should be telling/doing something so that I can smile. I really can't put on a fake smile.I'm a friendly person when people talk to me. I even thought about becoming a police officer because they don't have f***ing smile at all. In my 2 previous jobs my employers(both men) asked me why I wasn't smiling then then they laid me off after. I want to tell them "see, that's the reason I'm not smiling because I don't feel secure,I'm worried about my future." Geez!