There are a number of possibilities, which depends upon what methods of interaction we are looking at.
For example, with direct contact, we can estimate a probability of life along side how possible space travel is. Perhaps space travel isn't easy or fast at all and so there is plenty of life, but it is mostly stuck to its solar systems and maybe a few neighboring stars. Overall, given that we can send and receive signals much easier than we can send and receive space crafts, I think this isn't as useful a metric.
The better one is that we don't see signals from other life elsewhere, but this still has to be measured by how likely life elsewhere would be able to see our signals.
Lastly, there is the matter of what it means to be rare. Say only 2 or 3 planets in a given galaxy end up developing intelligent life, is that rare? Given the number of galaxies in the visible universe, that is hundreds of billions if not trillions of planets with intelligent life. Yet with only 2 or 3 in a galaxy, it would be easy for us to not see any signs because maybe we are the only ones in our galaxy or our galactic neighbors are on the other side of the milky way and we have no technology to communicate, nor will we for the near future. Hundreds of billions of intelligent species can be considered both rare and not rare given the sorts of scales we are talking about.
Also other edge cases, like maybe intelligent life is common enough but it tends to rarely progress past a certain point of development due to wiping itself out. Personally, every explanation I've heard or can think of has some sort of unpleasantness to it, much like the quote that says either we are alone or we aren't alone, and both ideas are scary in their own ways.