Should we spread life?

I cringe every time people panic about possible contamination of Mars. Why is this bad?

What if we find out Mars is a sterile environment? Would we pout and ignore the barren wasteland?

We work hard to bring life to deserts, we plant trees where there are no trees, all over Earth we spread life and seem to infect everything with it. We touch otherwise pristine objects, covering it in bacteria and viruses. We spread life everywhere sometimes intentionally, sometimes on accident.

Why on another planet is this a bad thing? On our planet we've been fortunate to have perfect conditions for life to not only develop, but become staggeringly complex. I think we owe it to the universe to spread the gift of life to everything around us. We've become intelligent enough that we are beginning to scratch the surface of creating new life so why not engineer a couple of slime molds that would thrive on Mars? Why don't we create some nematodes that burrow deep into the moon? Why not create some bouyant life forms that can float through the gas giants?

What if we are alone in the universe? This shouldn't be depressing, it should be something exciting. We would have the opportunity to become the progenitors of all life everywhere. We might find planets that do have life but nothing intelligent... why not enlighten the planet?

If the universe is barren, I believe it is our duty to spread life throughout the cosmos.