This has been asked and answered many times, from an evolutionary point of view one of the best answers appears to be kin selection:

Past research has shown that the fa’afafine are much more altruistically inclined toward their nieces and nephews than either Samoan women or heterosexual men. They are willing to babysit a lot, tutor their nieces and nephews in art and music, and help out financially-paying for medical care and education and so forth. In a new study, the scientists set out to unravel the psychology of the fa’afafine, to see if their altruism is targeted specifically at kin rather than kids in general.

They recruited a large sample of fa’afafine, and comparable samples of women and heterosexual men. They gave them all a series of questionnaires, measuring their willingness to help their nieces and nephews in various ways-caretaking, gifts, teaching-and also their willingness to do these things for other, unrelated kids. The findings, reported on-line this week in the journal Psychological Science, lend strong support to the kin selection idea. Compared to Samoan women and heterosexual men, the fa’afafine showed a much weaker link between their avuncular - or uncle like - behavior and their altruism toward kids generally. This cognitive dissociation, the scientists argue, allows the fa’afafine to allocate their resources more efficiently and precisely to their kin-and thus enhance their own evolutionary prospects.