There are three motives for helping others. Enjoyment, Pity, and Compassion. I think these are exhaustive. Enjoyment is the motive for the sake of oneself. Pity is the motive for the sake of the other with regard to their difference. Compassion is the motive for the sake of the other with regard to their similarity. Let these stand as definitions of those three terms, rather than arguing over whether I have rightly described Pity etc.
First, enjoyment seems to be a straightforwardly selfish motive. I do not think helping others should not be enjoyable, as if there was something wrong with enjoying helping another person. Rather, I think it is a terrible motive. We should enjoy it because it is good; not think it good because we enjoy it.
Second, pity seems to be a less straightforwardly bad motive. For one, I do not think it actually motivates real help. I suspect that pity leads us to try to satisfy our guilt. It is the dominant motive advertised for helping others. "We have so much, they have so little, let's help them!" There you go, feel guilty for having stuff, dump enough to assuage your conscience on the poor people, and run off without considering their actual concrete situation. Moreover, if you were in their shoes, would you want to be pitied? Isn't pity annoying? Pity accentuates the difference, creating distance, loneliness. I have, I am okay. You don't have, I can help you. Others thereby become projects, objects to be raised out of poverty.
Note, however, that there are real differences between people which need to be taken into account when helping others. I am not saying that we ought not account for difference. We are different and that is, prima facie, good (depending, of course, on the difference). What I am arguing against is being motivated with respect to that difference primarily.
Rather, compassion seems to me to be the proper motive for helping. We help, not because we have, but because we might not have. We are utterly dependent on others and might become further dependent on others at any moment. We need help, and thus we see in others situations, not some instance where I have and they have not, but where there is a person who might have been me, and I act with respect to them as I would act with respect to me if I were in their situation. I take their need as one which might've been mine. I see that their need might one day actually be mine. We no longer feel particularly good about helping another, but rather see it as the done thing--if I hadn't, who would've? What if it had been me?
This is my thesis: we help best out of a sense of our own need. We give most out of a sense of our own lack. We forgive most out of a sense of our own depravity. Yes, this is paradoxical. Look around and see, though: it is kinda how it works.
Not a study, but... http://themetapicture.com/what-this-man-does-with/