Reason

Yellowstone becomes a flood basalt.

Carbon-coated asteroid on an incidental orbit collides with Earth.

Gamma-ray burst from a distant star sweeps across half the surface of Earth.

The end-of-life scenario for Sol, where it increases in both size and power output, heats Earth just 40 degrees.

What all these events have in common is that they are inevitable. Given a long enough timeline, these all WILL happen, killing most or all life on the surface of the planet, either way ending life as we know it.

Among other things, this means that, in the very long term, conservation is meaningless.

Penguins may be the cutest animals. Crows, dolphins and great apes are the other sapient lifeforms that we know of. Bowhead whales live so long that they can remember the North Atlantic whaling industry. But the fact remains that if a low-visibility iron asteroid the size of Manhattan decides to turn itself and most of the Baja Peninsula into a bolide, none of those animals would matter anymore. If, starting tomorrow and for the next thousand years, Yellowstone decides to open up and cover the entire Pacific Northwest in molten rock, the Deepwater Horizon disaster isn't going to be remembered by the alien archaeologists that discover our sooty corpses.

The simple and important fact is that life exists in the spaces between world-ending catastrophes.

Conservation is important while we're here, but is, in the very long term, pointless. Whether or not you recycle or turn the lights out when you leave a room will factor very little in the ultimate end-state of the Earth.

Of course, I don't mean we should extract the marrow from the bones of all polar bears while we still can. Conservation remains important for a simple reason: this is our only home.

My point is this: we have to find another home. The most common reaction I hear to this idea is "Oh, so we can just destroy another planet?" That's stupid for 3 reasons:

1. Saying that is essentially equal to saying "We should just kill ourselves right now."
2. Another planet would be equally susceptible to the very same end-state I'm trying to warn us about. Even though it'd be another 'basket' for our 'eggs,' it'd still be at risk.
3. I'm not even suggesting we find an already-habitable planet and go there.

We're a smart species! If we took all the money we spent on war and waste in the past 20 years alone and spent it instead on medicine and NASA, a Martian colonial scientist would have cured cancer already.

We don't even need to go to another planet! The craft necessary to transport extrasolar colonists on that scale would necessarily be self-sufficient for extended periods, and more importantly, compared to a planet, mobile.

In sum, because life on Earth WILL end, one way or another, we NEED to leave. We MUST become creatures of the stars.