Look at the universe we find ourselves in. It's a hostile place out there yet here we are, cozy and comfy on this warm blue marble of a world. There's water all over the place and tons of stuff to eat. Life is good.
Beyond this, we can't help but compare ourselves to the other life forms on this planet. They have it great, too; don't get me wrong. But we're the lucky ones, the creatures who can understand our world and manipulate it to improve our situation. Better yet, by studying the world around us we've discovered awesome things: general relativity, quantum mechanics, evolution, dark energy, dark matter and black holes! Woot!
As a species, we're sitting pretty (while ruining the planet, but that's another post). A human can have a very rewarding life on this planet. Yet somehow, all this is meaningless if there's no heaven? What is wrong with religious people?
John Lennon's lyric -- Imagine there's no heaven -- is ultimately silly. You don't need to imagine anything. Just open your eyes and what you see is a world without a heaven. You're in it, kid. Congratulations!