In this paper, we investigate the cooperation gain from secondary relaying in spectrum sharing networks, where the secondary system helps the primary cellular network by relaying the traffic of cell edge users. In cellular networks using frequency reuse, only a part of the total spectrum band is assigned to each cell, according to the reuse factor. In one cell, the unused frequency band seems to be wasted in the sense of spectral efficiency. Utilizing this unused spectrum in each cell, the secondary users relay some primary packets as well as transmit their own generated packets to the secondary access points. Through the secondary relaying, the performance of primary users at the cell border is increased and this leads to the primary capacity improvement. The results show that the cooperation can compensate the increased interference by spectrum sharing, giving benefits to primary networks.