Well, it doesn't quite. 예뻐서 in dictionary form is 예쁘다. 예쁘다 means "to be pretty". When conjugated in the simple present tense, informal polite, it's 예뻐요 (x is pretty). However, 예뻐서 is 예쁘다 plus the grammar pattern 아서. 아서 is used for cause and effect. It means "so, because, since". So 예뻐서 means "because (you're) pretty".

so 예뻐서 can also be the begining of a sentence. such as, because your 예뻐서...blah blah blah. Right?

고맙습니다