In mobile ad-hoc networks (MANET), multi-hop communication relies on a give-and-take relationship among users. For stimulating users to be more cooperative in MANET, an incentive mechanism that offers electric power in exchange for relaying packets by using wireless power transmission (WPT) technology has been proposed. To evaluate the availability of MANET and this incentive mechanism, it is important to reveal factors affecting to the user's willingness to relay packets. In this paper, we investigate and analyze user's willingness to relay through qualitative evaluations. On the basis of the analysis, we model user's willingness to relay packets depending on the residual battery level and user's characteristics. Finally, we also confirm that the electric incentive mechanism can stimulate reciprocal cooperation among users in MANET.