Nobody says with certainty that life exists. But they do say the probability is so high that life in some form exists elsewhere, you might as well be certain.
As for not sharing the universe: what a depressing thought.
Why is that depressing? It's not like we would actually be ALONE. We have each other. We don't need Vulcans or whatever else to keep us company and suck up our resources.
When you put it that way, it doesn't actually matter. There's always going to be someone competing for resources. The only difference is whether they have hands or tentacles.
As for not sharing the universe: what a depressing thought.