Why am I a doomer? The key question facing humanity post-peak oil is "What is the decline rate going to look like?" Right now nobody has a good answer to that because we're still arguing about whether we've even peaked or not. But ignoring that problem for a moment, whenever peak does arrive, the decline rate question becomes the big one.
The key problem becomes adaptation rate of alternative technologies versus the decline rate of fossil fuels (because natural gas appears to be at or near peak also and coal is not really that far off). If we can adapt as fast or faster than we decline then we may even experience a "boom" of sorts. If we cannot adapt as fast or faster than the decline rate, then we have problems. The exact nature and scope of the problems are not clear and it is not even clear that worse declines would lead to worse problems. Worse declines might trigger an emergency mindset sooner, thus saving us from ourselves, for example.
The problem that most "doomers" see is that historically, no energy shift in the history of industrial society has occurred in less than 50 years, or about 1.5%-2% change annually. Decline rates are already posited to be likely larger than this with the lower bound on decline rates most frequently expected to be around 4% and even industry experts seeing the possibility of 8% declines. Clearly, society has never faced this sort of problem before and therefore this problem is very likely going to require extraordinary measures rather than "business as usual" methods. So far, there has been no "crisis" mindset adopted by any nation in the world and therefore we remain in the "business as usual" mindset. This is what gives many "doomers" concern.
It is important to note that we have the technology to solve these problems but that using the technology we have would result in a society that looks different than the one we have today. Solving the energy crisis would mean denser housing, more transit oriented development, electrification of transportation, and a vast array of other effects that would fall out from those primary changes. Such a society would not necessarily be a bad place to live, just different than the "happy motoring suburbia" we have today, and maybe even better.
The negativity often expressed here is an outgrowth of the lack of response by national governments coupled with the awareness that the problem is already expected to be larger than normal modes of operation can handle. Also, note that many here refuse to accept that we also have massive concurrent problems with climate change, loss of arable soils, fresh water depletion, loss of biodiversity, and overpopulation. These problems are compounded by the extraordinary nature of the projected decline in fossil fuels. It is my belief that all of these problems are technically solvable but technology alone never solved anything by itself. Problem solutions involve politics, i.e. human interaction. If a political solution is not forthcoming then there will not be a solution, regardless of whether technology offers us such a solution or not. So what we need and are not yet getting is a crisis mindset on the political landscape to deal with these sorts of problems. Even in the European nations the notion is "business as usual".
Now, I will say up front that I cannot prove that the "business as usual" methods will definitely fail but, based on history, I can find no example where these business as usual methods have solved problems like these ever before. So by not adopting a crisis mindset througout our entire society, we are taking a gigantic gamble and it is a gamble that history suggests we are going to lose. I am not aware of a single prior civilization that continued to do things the same way and which faced one or more of these kinds of problems that did not collapse. Those civilizations cited by Tainter and Diamond that did survive serious crises, did so by changing urgently. Thus, until I see our society deliberately choosing to succeed, I must assume, based on the historical record, that we will fail (and therefore collapse). Given that many population biologists are of the opinion that we are in serious population overshoot only enabled by our civilization's existence (and dependence on fossil fuels), the loss of that civilization must be presumed to be accompanied by serious loss of life and disintegration of the social structure, as has happened repeatedly throughout history.