hubris
mankind as a species is sorta full of itself. We have a false sence of entitlement, and feel nature should bow to our wants and needs. We use the planet irresponsibly, with no consideration for the big picture
If we somehow survive ourselves long enough to leave the solar system, any external life we encounter will almost certainly be completely foreign to us. With our current attitudes, we wold most likely see it as more of an animal than another intelligent being. Even if we recognized the intelligence of the other species, our perception of them would cast them as immediately inferior, just because they are not us. This perception will pierce through any diplomatic attempts, which will most likely lead to hostility, and possibly war.