-they have a religion and want to bring it to us, destroying those who won't believe.
-or, alternately, their religion requires the sacrifice or consumption of sentients, and they're overjoyed to use us instead of each other.
-they decide to enslave us for our own good (taking up the gray man's burden).
-they've found so many other civilizations and sentient life forms that they see no value in talking to yet another.
-they have technology that is fueled by souls and see us as merely a potential energy source (yeah, that one's kind of out there).

__________________
"You miss 100% of the shots you don't take" - Wayne Gretzky