I used to look up at the sky and go "I wonder if there's other people out there?" I believed there was - early reading of my father's SF and watching Star Trek TOS made me think so. And certainly the findings of incredible numbers of exoplanets by various space telescopes makes you think that there's certainly got to be OTHER planets harboring intelligent life - and they're out into space exploring.
Now, however - I've got my doubts about this one. Yeah, it's kind of funny - but...
Observing the 'green' movement... I'm struck by how they're determined to drag humanity back into an imagined time where man lived in harmony with the earth. (They did, you know. You lived a short life, were locavores, didn't impact the CO2 count much, died young, and the worms harmoniously disposed of your remains.) The concept that people should live comfortable lives, LONG lives, that they should be able to do what they want, eat what they want, have sufficient energy on tap to power their dreams seems absolutely horrific to them. The concept of a future where people are happy and comfortable isn't one they want to come into being.
Not consciously, of course. You'll get separate groups, with 'similar' goals, looking to have their say in creating a better future.
You'll get 'sustainable' energy groups, looking to get away from dirty forms of energy. (And what's 'dirty' seems to be variable.) You'll get others, dedicated to protecting the environment. You'll find some who are looking at 'sustainable' agriculture and aquaculture. They're passionate - passionate to a point where they'll stomp all over each other.
You've got anti-wind, anti-nuke, anti-solar activists, you've got GreenPeace and 'sustainable activists, you've got loads of folks who are totally focused on their missions.
And then you've got the lawyers. They don't care what the client believes, as long as they can get money out of it.
But they all require a certain level of civilization and technological progress to sustain their activism. (Except the lawyers.)
So what I'm theorizing is that above a certain level of civilization - say, about a mid-1940's level of comfort, sanitation, electrification, food production and medical advances - the leisure time is there for the activist class (for lack of a better term) to develop. And they're poison to progress past that point, looking at the past and going 'Well, if only we could (insert fantasy here - revert to trains, return to windmills, go totally locavore) we'd do just fine, but this modern stuff (like nuclear power, planes, cars) is so horrible..."
Where would we be power-wise if - after Three-Mile Island our 'leadership' had gone "Okay - we can do better than this, and we will. We know what happened, we know what to do", and then had done it? Instead, the activists got a defacto moratorium in place that pretty much stopped any new nuclear power plants, and pushed to stop a lot of others. (San Onofre is a decent example of that.) You get folks saying that all nuclear power should be scrapped, because of what happened at Fukushima Daiichi, when an earthquake and tsunami an order of magnitude larger than the system was designed for occurred, and wiped out the power going TO the plant (which was used to cool things) and the backup generators.
You don't get people going "Okay, we've learned from what went wrong - we had everything pretty much right, and we won't make those mistakes again." Or at least, not anyone the media will listen to.
And don't get me started on the planes vs cars controversy...
Civilization is a fragile thing, held together by trust. As seen in the Roman empire - when the trust is gone things fall apart.
When that trust is eroided, or missing completely, there's little to nothing that can be done to replace it. See the USSR as an example of that - the poor saps trusted the government, which turned out to be untrustworthy.
The activists don't trust the folks who keep their lights on, who provide the toys for them to play with, who grow and transport food. How much credence should we give them - if what they'd ostensibly want would be to slap us back into a Middle-Ages style agrarian lifestyle?
(And with the lack of easily available coal and iron ore and the like, it'll be hell to rebuild if there were to come a technological crash.)
So my guess is, to explain why other civilizations aren't crawling around the galaxy, is that they get to a point where they're just about to make the leap to space... and their activists pull them back. There's a point where you can do it, where if you've got the resources and the will you can make the leap outward - and if you don't ACT when you've got both, the window of opportunity closes. Perhaps temporarily, perhaps permanently... but it closes.
And I think we're perilously close to seeing that window close ourselves.