Pema Chödrön's The Places That Scare You, in Chapter 9 discusses Tonglen, "exchanging oneself for others" — taking in the pain of someone else and sending out happiness. In the middle of the somewhat-mystical discussion:
Doing tonglen for another person ventilates our very limited personal reference point, the closed-mindedness that is the source of so much pain. To train in releasing our tight hold on self and to care for others is what connects us with the soft spot of bodhichitta. That's why we do tonglen. We do the practice whenever there is suffering—either ours or others'. After a while it becomes impossible to know whether we are practicing for our own benefit or for the benefit of others. These distinctions begin to break down.
^z - 2015-02-13