A: Practicality, aesthetics, and a bit of good ol' narcissism.

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 For practicality, what's good for the environment is good for us. It's cheaper in the long run to prevent environmental catastrophes than to take care of the aftermath. This is also comparable to public health because stuff smog can cause lung issues, chemical pollutants can cause life-long health defects, the list goes on. 

It's also cheaper to switch to electric or renewable energy sources (and will create thousands - if not millions of new jobs) and will become even more cheaper as innovations occur and is more widely used. 

Then it's a security issue that people have widely considered the root cause of the Syrian Civil War and to an extent the rise of ISIL. 

Then Pacific Islanders will lose their ancestral homes due to rising sea levels - which breaks my heart more than anything. 

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For aesthetics, the Earth is just gorgeous - from the old growth forests of Japan to the plumage of the bird of paradise, there's very few natural things on this planet that are hideous. Some things may be scary - like what's lurking in the depths of the ocean but that just adds a certain intrigue to this planet. I can get why transcendentalists say that God manifests itself in nature, because some of the most incredible pieces of art are found not in museums but en plein air. 

Then compare an old growth forest to a coal mine or a forest of wind turbines to a billowing smoke stack. The contrast of beauty and elegance is immense - and beauty is basically the most important thing in the world lbr. 

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Now for narcissism...

Someday I would like to have children. I know that may be a hypocritical and selfish statement to make considering I'm aware of the huge impact that a child has on the environment AND the rough state the planet is in. But I want to be a mother, it just seems like the most rewarding thing to be a parent.

Therefore, I want to make the Earth a better place to live in so that my children and my children's children have a wonderful place to live and to never be able to see the beauty of the Amazon Rainforests or see the glaciers of the Antarctic.

There's also a lot of scary (but very plausible theories) that melting glaciers will lead to ancient disease outbreaks (with no cures) and that there will be so much carbon in the atmosphere that it effects the way that plants produce oxygen - suffocating the planet.

I want to avoid future generations from hating us as much as possible and want us to thank us for FINALLY making a change to a greener, more sustainable way of living.

Also, if an alien civilization finds the Voyager One spacecraft, play the golden record and fall in love with the Earth and human civilization - they should have the chance to come here to experience first hand. It'd would be sad for them to come all the way here just to find a desert planet.
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The future of this planet on the surface looks very bleak, however if you dig a lil bit deeper you'll find that there's a lot of good innovation and progress happening every single day. Think carbon sequestration, 3D printing of coral reefs, Solar roof panels, cities running on 100% renewable energy, the list goes on. If we work hard enough, we can definitely mitigate this horrendous future.

I truly believe that everyone should be an environmentalist.