Jacob Neusner, Invitation to Midrash: The Workings of Rabbinic Bible Interpretation (Atlanta: Scholars, 1998) 10.
Pirke Avoth is a part of the Mishnah dating to 250 C.E. It quotes Hillel (first century C.E.):
If I am not for myself, who is for me? And when I am for myself, what am I? And if not now, when?
This sounds like a reasonable contrast to those who talk like we should completely sacrifice ourselves. Of course we should be for ourselves, but we shouldn't be only for ourselves. "If not now, when?" I read in a sermon that this means there's no time like the present to do works of healing the world. That sounds like a good balance between self-interest and altruism.