reasons why we prefer live-ins?

I think it's a double edge sword... on the one hand, it's great (or so they say) to find out if you are really compatible enough to make the relationship work. On the other hand, people evolve and you can never be 100% sure, so living together becomes a permanent thing (until the breakup). In all, I think the trick to a long lasting relationship is commitment... and "testing the waters" means you can always pack up and leave, which makes it easier to do so.