The process by which women came to donate milk via online milk sharing networks was explored via a questionnaire administered to 97 peer milk donors. Seventy-one respondents stated that they were motivated to donate milk because they wanted to help someone. Many described milk donation as an empathic response to women with insufficient milk. Seventy-four respondents donated milk that they had previously expressed but did not need. Their desire to ensure that their milk was not wasted contributed to their decision to donate. Fifty-one respondents expressed milk specifically for donation, including 20 donors who initially donated previously expressed surplus milk but then expressed milk specifically for recipient peers. The motivations of peer-to-peer donors are the same as those reported for women donating to a milk bank. Respondents who donated previously expressed milk had originally expressed so they had milk when separated from their baby, to manage an overabundant milk supply, because their baby was unwilling or unable to breastfeed, in case of emergency, and to maintain milk supply. This study is the first to clearly identify that some women express milk because they believe this is required for milk supply maintenance. Peer milk donors appeared satisfied with their experience of donating milk. The importance of altruistic motivation should be considered in discussions of the desirability of financial compensation for milk donation. Further research is needed on why women express their milk and whether such expression is beneficial to them.