"When I first went to India, I met with a swami there, a member of a monastic order. I told him about my work and how I thought it would be important if reincarnation could be proven, because it may help people to lead more moral lives if they knew they would come back after death. There was a long silence, a terrible silence, and finally he said, 'Well, that's very good, but here, reincarnation is a fact, and we have just as many scoundrels and thieves as you do in the West.' I'm afraid that rather deflated my missionary zeal."
I was thinking particularly about the difference in how one might approach their life in a "this is all you get" (or "traditional western") mode vs. a "this is just your current spin of the wheel" (or "eastern") one.  I mean, at the end of the day, particularly given the power of rationalization, people are pretty much going to do what they're going to do.
Take "chasing a lifelong dream" for instance. In theory, if you believe you only live once , that should inspire you to go for the gusto! ...Unless you are so fearful of screwing up your one chance that you get paralyzed by indecision.
On the other hand, if you believe you will go around again and again until you get it right, that should inspire you to take risks and go big because failure is an illusion! ...Unless you get distracted or just putter around forever because there's nothing pushing you to strive forward.
In the end, we (being humanity writ large) don't really know (although I suspect most individuals have pretty solid views on the subject). Regardless of how the game actually plays out, what people do still boils down to individual character. A saint is going to be awesome to/for everybody because that's what they want to do, regardless of whether heaven is waiting for them or not; an asshole is going to be an asshole whether they're just plain dead at the end or they'll have a karmic debt on the next go-around.
So, yeah. As strange as it seems, it looks to me like the afterlife and worldly morals/ethics have surprisingly little to do with each other, regardless of what the preachers say.
 I have read (but don't know how true it is) that early Christianity embraced the concept of multiple lives, but that it was basically "edited out" by church leaders in favor of the "heaven/hell forever" model in order to make it easier to control their followers. It was a totally fatuous thing to do, if so, but given the way humans are sometimes, it also wouldn't surprise me.
 Either because you get one shot at life and go to heaven/hell, or because we're all bags of electrochemical reactions that are effectively self-programmed robots who just think we have a consciousness because we've evolved to tell ourselves so. Pick your existential horror of choice.