Why should anything survive at all?
Surviving is the process which a living organism takes part in to prolong its own bio-mechanical body through time. Predators such as the crocodile try to survive by hunting specific prey and eating it, digesting its flesh and ultimately absorbing its nutrients, vitamins and proteins. Prey such as the gazelle (hunted by the cheetah) try and survive by consuming vegetation and water, they also try and survive by avoiding predators. Most living organisms survive through consumption of other chemicals and through avoiding predators.
This begs the question; Why do living organisms survive? It is programmed within them to survive. It is innate. They are born to survive. Therefore we must look at the programming of living organisms to fully grasp this innate urge to survive. It so happens that all of the diverse life on earth have a similar program, all of these programs use the same code and the same information infrastructure to build organisms. This code is now called the genetic code. The information infrastructure is how these codes are 'read' out by certain (RNA transcription ect.) molecular processes to build proteins and cellular tissue. Each living cell (upon development) of a living organism needs instruction on how to build itself and how many, these instructions come from DNA which contains genetic information. In cells strands of DNA form into chromosones with four cistrons (legs of chromosone).
It so happens that these genes which are long lived units of chromosone, also seem to 'want' to survive. 'Want' is a word which connotes a conscious decision for something in particular, we must always remember that genes do not think, they are merely units of chromosone containing biological information. So, genes want to survive. Genes (assuming all other integrated genes remain the same- as genes work together) which promote longer legs in a Gazelle may be favored in Gazelles, as longer legs could increase their stride length which could increase speed to avoid predators. Those Gazelles with longer legs may have a better chance of survival, which means they could reproduce more than those Gazelles which died due to shorter legs. This means that over time more Gazelles will appear with longer legs. The gene which promoted longer legs will eventually dominate the other alleles in the gene pool. It will have increased it's own chances of survival.
So organisms survive merely because their bodies have enabled them to persist in certain environments and they are programmed (by their genes) to want to persist through time. This is due to their genes wanting to survive.
However why should genes 'want' to survive. They are merely a complex accumulation of chemicals and molecules... why should they want to persist as they are? This question could be asked ad infinitum on smaller and smaller scales. Genes survive merely because they do. The physical nature of those chemicals and molecules dictates that they should, the laws of physics dictates that they should survive.
It becomes extremely confusing to many people who wonder 'why' we, life and genes survive. It is ultimately how the physical world over time has developed. It becomes confusing because there seems to be no conscious entity driving things forward... evolving them. The universe over time has enabled this to happen, by chance life exists and obeys a program which forces units of information to survive which in turn forces biological cells to survive and at last for us, living organisms to survive.
Why should anything survive at all? It is how the universe has developed. It has led to complex life. Nothing needs to survive, it just happens that things do. It is a beautiful event.