You should be positive when you feel like it and only sometimes when you don't. The "when you don't" might include when you are looking for a job and feeling down because you haven't found one or found the right one. This would be the Law of Attraction at work. Years ago, I was having career problems and feeling very down. My job was low-paying, my kids and I were living in a basement apartment in a not-so-good area, and I was driving an old brown car. I despise old brown cars. I dressed like somebody with a crap job and an old brown car.
One day I thought to myself, enough of this, I am Somebody! I pulled my better clothes and shoes out of the closet, fixed up my hair and held my head up. Within six months, I had a better job in the career I loved so much. Coincidence, I don't think so.
But on the other hand, I had a friend whose psychiatrist told him to "let his feelings out." He did, he truly bitched and ranted over the smallest things that never made a hill of beans. I had to break up with him, and so did his other friends. He is one lonely man. So I think there should be a balance here.
If you are aware of life, life will tell you when it's ok to have a down time. A real person can't be "up" all the time.