Bonobos

Some scientists say that given the size of the Milky Way galaxy, there are surely other planets with intelligent life. Because...there MUST be!

That's not a good reason. There's the famous Drake equation, of course. It looks like this:

N = R4 x Fp x Ne x Fl x Fi x Fc x L

According to Wikipedia, the problem with the Drake equation is ...

"... that the estimated values for several of its factors are highly conjectural, the combined multiplicative effect being that the uncertainty associated with any derived value is so large that the equation cannot be used to draw firm conclusions."

In other words, the existence of alien life is anybody's guess, wrapped inside a fancy formula. It could well be that there is no other planet with intelligent life. In fact, it may well be that there is no other planet with any kind of life at all. We don't know.

If we are the only intelligent life that our galaxy has brought forth, I think it's a pretty sad state of affairs for intelligent life. We've got one planet that is livable and we've screwed it up pretty badly.

Maybe intelligent life is an experiment that is now beginning to fail. Maybe in a million years, bonobos will have evolved to our level of intelligence and will take our place. That is, if we don't kill all of them first.  According to Wikipedia, bonobos "seem to prefer sexual contact over violent confrontation with outsiders." That pretty much dooms them, especially around humans. 

Sorry, bonobos. You had so much promise.