Let's imagine a story where the "aliens" leave a lot of super-advanced technology somewhere in the solar system. Scientists spend years excavating this technology and while it seems too advanced for us -eventually it helps to progress our level of civilization. However, after years of research, scientists have a hard time finding out who the aliens really are.
Then, in a plot twist, we learn these are actually future humans from another universe. They came here not just for a "roadside picnic" but because their universe has more life than our own...in their version of the Solar system they had intelligent life on Mars and Venus (the composition of these planets is much different, as with their sun), and that means they were constantly attacked and exploited, not just by intelligent life but also many alien bacteria from other worlds.
So while we struggle to find ANY life in our Galaxy (even microscopic extremophiles) they had contact since antiquity with other civilizations (some were peaceful and didn't intervene with their development). With the invention of the radio, they heard messages from other star systems far and wide. When they progressed to the space-age, they even found some primitive lifeforms on the moon.
With all these lifeforms that existed on almost every planet-and in most star systems-many big empires emerged that were fighting each other. At the end of the XXII century, Earth almost lost a war that destroyed most of humanity. This is why they think our universe is much safer, and theorize that life must be extremely rare here.
Now, we are talking about alternative evolution here..the alternate universe has different laws of physics and life is much more common.
So...do we have any theoretical idea of what needs to be done with the laws of physics to make life more common? Tweaking certain laws, making stars closer or farther from each other, you name it.