The chances of life developing are thought to be extremely small and that the formation of it on Earth was a cosmic fluke that is unlikely to ever happen again. It is thought that the chances are so small that we are likely the only life, and within the observable Universe.
1) The Ribosome Argument No life has ever been observed to exist either elsewhere in the Universe or seen to form in the laboratory. Though the building blocks, Amino Acids, have been seen to form, the next step, for the formation of Ribosomes is thought to be significantly higher. Ribosomes are the molecular machines by which RNA is formed by stringing together these acids. It is thought that RNA or something similar to encode the instructions on how to grow and reproduce are essential for life, but the only way that Ribosomes have been seen to form is by the use of other Ribosomes. It causes a chicken and egg paradox where Earth is likely to have been a fluke where simple Ribosomes we're able to form via chemical reactions. This has never been seen to occur and is thought to be orders of magnitude more difficult compared to Amino Acid formation. 2) The Great Filter Following on from the Ribosome argument, this refers to something that exists in the Universe that prevents life from flourishing. For most biologists and palaeontologists, this is thought to be the process of Abiogenesis, the process by which non-living matter becomes living. It is widely considered to be one of the bottlenecks in the formation of life due to its seemingly low probability of occurrence.
Though we have been looking for signs of intelligent life for ~50 years or so, we have only extremely recently obtained the ability to observe some exoplanets. Very soon, we will have the ability to analyse the atmosphere of exoplanets for signatures of life such as seasonal methane and oxygen cycles. So till we actually have the ability to look for signatures for simple organisms, we cannot use the fact we haven't found any as evidence against their existence. Though we currently have not observed a process by which ribosomes can form naturally, this does not mean there is not one that can occur fairly easily. We have only been studying the conditions that were present on ancient earth for a century or so. Nature had hundreds of millions of years. ADD ARGUMENT AGAINST GREAT FILTER
There has been no evidence of molecules evolving beyond the point of Amino Acids Ribosomes, which are required to combine Amino Acids, itself needs Ribosomes to be created, the only other way is through luck that the exact correct chemicals were mixed in the right way to produce this extremely complex molecule.
Rejecting the premises
The fact we are here shows that I can happen. It is highly speculative to guess at the likelihood of Amino Acids combining- This argument also applies to the idea of primitive Ribosome formation.