The answer to this can be found in Chapter 12.
The monster has run off from Frankenstein's apartment and he has been living in this village. He has been watching all the people going about their business and he thinks that they are quite beautiful to look at. At one point during this time, he sees himself reflected in a pool of water. When he does this, he sees that he is hideously deformed compared to them.
He is really quite taken aback by the way he looks. He cannot believe that he looks so horrible and he gets quite depressed because of it.