Edit for a TL;DR - The basic idea is that, if we don't see evidence of intelligent life in the universe, it must be because star traveling species must be rare. If we find life evolving twice in one solar system, then the early parts of life evolving must be relatively easy - which suggests the later parts of life becoming technologically advanced are the hard parts. In other words, life on Mars may mean that there may be more problems ahead of us than behind us.