Life is not mysterious, where it can exist, it will exist. Since there are billions of galaxies, each with thousands of earth-like planets, it is a safe bet that there is life out there.
It is likely that over the last 10-15 billion years, there have been many civilizations which have evolved. Our sun is only 5 billion years old, since the supernova. Our civilization is only centuries, maybe millenia, old. There have likely been civilizations which have evolved hundreds of thousands, millions, or even billions of years ago. Some of these civilizations have probably been stable.
If stable civilizations have existed for millions of years, they have had time, even with sub-luminal transport, to propagate throughout the universe. This makes it more likely that any planet capable of supporting life probably does support life. Earth may be a long-term experiment... What else would you do if you had hundreds of thousands or even millions of years to play with in a stable, boring, civilization?
For a civilization to be stable, it would need certain characteristics. The citizens would need to be rational, and not killing or harming one another arbitrarily. There would probably be minimal superstition, since superstition seems to be strongest in primitive cultures. Since most mutations ( over 90% ) are deleterious, there would need to be methods of regulating the gene pool. The birth rate would have to equal the death rate. The civilization would tend to become fragile ( sensitive to change ) unless steps were taken to ensure that the gene pool remained vigorous. They would probably be quick to breed out primitive characteristics such as aggression and sexuality since both of these affect total population significantly.
Breeding out ( or otherwise removing ) characteristics also reduces the vigorousness of a gene pool. Faced with a tailored environment due to technology, there is less stress on the population. The population then becomes sensitive to change, i.e., the gene pool is less vigorous.
A stable civilization is likely to exist in a delicate balance, so that any threat of change or instability could be catastrophic. It could be speculated that they would be very cautious in their dealings with primitive species such as humans. It could also be speculated that primitive species could provide an opportunity to re-inject vigor into a gene pool. Why should their own population need to suffer through diseases and stresses?