Absolutely not. I've dealt with a lot of pessimistic, depressed people--I've been clinically depressed, as well, in the past--, and we can be the most closed-minded, whiny, oblivious people out there. I think that's exactly how depression actually works: one is enslaved, kept away from experiencing the world as it is, by their extraordinarily reduced perspective, which is seemingly unescapable to the depressed individual.
I don't think it's reasonable to conflate negativity--which leads to bad outcomes--with a realistic mindset--which takes into account potential bad outcomes. I find the opposite to be true, in my experience: the more positive and proactive a person is, the stronger they can become--and not crushed by a negative turn of events.
1) We have a perverse tendency to reinforce our existing views in new experiences. For example, if one believes he/she is not a very charming and likable persom, every opportunity to make a new friend may thrown away because the person doesn't have confidence to present themselves in an interesting way or the person may misinterpret another's neutral body language as a negative signal during a conversation.
2) Experience is the best teacher. Experience etches things into our subconscious. If negativity is etched in our brains then it takes a strong conscious effort to over turn it. This can potentially scary because the effort to experience a positive outcome may result is us being beat down further. But we have to see it for what it is, one experience, the next may end differently. Overcoming depression requires concious effort, hope and resiliencey. This is not something that happens overnight.
I have been mildly depressed in the past and sometimes it struck me how narrow my view of the world was in those days. Sometimes literally, it seemed as if I was looking through a tunnel and that I was more busy with all the negative thoughts that were going on in my head than with what was really happening around me.
This can get out of hand when circumvention becomes a lifestyle.