There was a good explanation several years ago. Imagine something needs to go from zero (beginning) to gazillion ("fair" value). It can't just slowly grow to gazillion in 2020 because as Risto said once everyone notices that it will do that, they will try to jump on the train before the others - human nature - and since no governing body can fix the price or close the markets, you get overexcitement runs (when you deviate much from your slow trend to gazillion in 2020), followed by busts, but on average it is pretty clear that we are, in fact, slowly and steadily climbing upwards.
Only bears can't seem to see that, but on the other hand, they do sleep in caves for like 90% of their lives, so there's pretty much everything else that they don't see as well.
I absolutely agree with this view, it's been my opinion for a long while too. I think it would be IMPOSSIBLE for a commodity like bitcoin to grow in any other way than through a series of increasing bubbles. It's a pattern that will essentially "continue until it doesn't".
BTW can't we be both short AND long term bullish?