Why Life?

I've been thinking lately about what 'life' is and why it's so persistent. If we define 'life' as something that grows, reproduces and expands it's territory, I think we have a pretty acceptable definition. My big question is 'why?'. Why does it 'life' have such a strong drive to create more of itself even when that process means the end of the individual? The male praying mantis needs to mate with the female. When the mating is over, she eats his head. Salmon swim thousands of miles to where they were hatched to mate and die immediately after. Even in those species that don't die after mating, both male and female will die protecting their offspring if required. Even humans generally follow this rule.

What is the purpose of this? Is there some force in the universe that demands 'life' spread as far as possible? Most forms of life expand to fill the environmental niche they can comfortably live in. That limits their growth area but not the need to continue creating more individuals. Humans are different though. We've come up with ways to make the whole planet habitable and now have a desire to spread to other planets.

Humans have sometimes been compared to a virus. Not a totally incorrect comparison, but one that over-simplifies how humanity operates. We care aware of the consequences of our actions, even if we choose not to avoid them. Virus's can't do that. But both operate on the compulsion to expand as far as possible.

So I as 'why?'. Why does life in all it's forms have this need?