I’m very happy to see this being discussed, and have enjoyed reading others’ answers.
Upon reflection, I seem to have a few different motivations: this was a surprise to me, as I expected to find a single overarching one.
a) Imagining another person’s experience, leading to imagining what it is like to experience some particular suffering that I can see they are experiencing. Imagining “what it is like” involves focusing on details of the experience and rejecting generalities (not “I have cancer” but “I am trying to reach down in the shower in the morning but can’t and the water is too hot”). Soon my train of thought goes to a more objective or detached place, and I think about how there is no real difference between me and the other person, that except blind circumstance there is no reason they should suffer when I do not.
There is an erasure of self involved. I imagine the core of my consciousness, the experiencing self, inhabiting the other person’s body and mind. From this one example I generalize; of course I should treat another person’s suffering the same as my own, because in the final analysis there is no difference between me and other people. That’s the altruism; desire for effectiveness is secondary and instrumental, not terminal.
b) Zooming out and imagining the whole of the world leads to imagining all the evil in the world. (Where “evil” is a broad term including suffering due to carelessness, due to misaligned incentives, due to lack of coordination, due to accident, etc.) It’s overwhelming; there’s a sense of perverse wonder. “The works of Moloch are as many and burn as cruelly as the white-hot stars.” This leads to a powerful feeling of being “fed-up” with the bad things. The desire for them to stop is like a very strong version of the desire to clean up an untidy room. It’s abstract and not connected to any one person’s suffering. This tends to be a stronger motivating force than a); if a) is empathy, this is anger.
Eliezer’s fiction is particularly good at conjuring this mind-state for me: for example, the “Make it stop” scene in http://yudkowsky.net/other/fiction/the-sword-of-good .
This mind-state seems more inherently connected to effectiveness than a), though effectiveness is still instrumental and not terminal. I want us to be making a strong/effective stand against the various bad things; when we’re not doing that, I am frustrated. I am less willing to tolerate “weakness”/ineffectiveness because I conceptualize us as in a struggle with high stakes.