Which view is best?

One's view of life could be either pessimistic or optimistic. A person with a pessimistic view of life see's the negative side of everything.
Example: Erica had $20 but bought a shirt that cost $10. She was upset because she only had $10 left.
She had a pessimistic attitude because she thought about it as only having $10 left when she just had $20. If Erica would have had an optimistic view she would have been happy that she still had $10 left after buying a shirt.
Some people have optimistic views of life while others have pessimistic. The reason is undetermined but someone can make suggestions. Maybe people look at things in an positive or optimistic way because its makes them feel better. Maybe they can't see the negative or dont want to. Maybe seeing the negative side is hard for those who can't handle the truth.
I don't think anyone really knows why people have pessimistic views of life except those who are pessimistic. Maybe they think about the worst that can happen so when something not as bad as they thought happens happen, its easier to deal with. It could even be that more bad things has happened in their life than good and thats how they see everything.
Say for instance your significant other says those three words "I love you". A person who is optimistic would love the fact of being loved while a person who is pessimistic would respond "for how long".
Which view is the way to be? Optimistic? Pessimistic? Who's to say? Can you be both? Who can decide which is the best view and wouldn't most people say the view that they are? What would I say? What should I say? Who am I or anyone else to decide which view is best?