Research shows that we learn by talking about what we read and teaching what we know to others. In the scaffolded setting of collaborative groups, students read together, make meaning together, and teach each other what they know. They develop a respect for each others' strengths and patience for each others' weaknesses. Each students builds self confidence as a reader and a learner. In this collaborative setting, students are not afraid to say they don't know and to seek help from members of the group or outside sources.
Reciprocal teaching works because it's what good readers do.