I think this is fine as long as you let other people be free to make the same decision.
So if you find someone who is upbeat and positive, and you like that a lot, but they find you too negative and break up with you, you have to accept that as a valid reason. Finding someone who complements your temperament is important, but you have to complement their temperament in a way they like too.
For me, for example, I'm generally fairly even tempered and positive. I would probably be a good balance for someone who is very moody, but I couldn't handle a relationship with someone tended to be moody and negative. I would find it too stressful to have to always be the upbeat one, to compensate.