First, with the innumerable galaxies and the innumerable stars and planets there are, what are the odds that another one like earth exists? There is one in this galaxy, why wouldn't be another one in the next galaxy?
If we believe what scientists believe that everything around us is just a result of a spontaneous series of events starting from the Big bang, the likelihood of another planet is high since everything came from the same "thing." Even if the probability of a system forming a planet capable of sustaining life is very, very small, the vast number of planets makes such formation probable.
Second, religiously speaking. God made everything around us. He made a rather huge universe and we are not in the middle of it, we are not even in the middle of our galaxy. Why would he make a huge universe if he is just going to populate this planet?
There are also scriptures that imply existence of other planets with life.
2 cor4:4; the use of "this" in the sentence implies there are others.
It is also a belief by the LDS church that there are other "earths" that this is not the only one God created.