To be motivated by the prospect of receiving a return is natural and normal. Who would pay for a product or service with the expectation of getting nothing for it? And in the case of doing a charitable work, while there might not be a conscious expectation of a benefit, there still is some kind of internal gratification attached. We consider someone who deliberately does something self-destructive to be a masochist, but even a masochist receives some kind of reward--evidence that they are the worthless individuals they believe they are, and administration of the treatment they feel they deserve.
I’ve heard Christians who were "righteously" appalled by the thought of serving God and people with the expectation of some kind of return or benefit, but when you look through the Bible, God always calls people or appeals to them with the promise of some kind of reward. Never does the Father or Jesus say, "Follow me, but you’ll get nothing out of the deal." There’s always a blessing--be it material, emotional, or spiritual--temporal or eternal--attached to God’s call. So clearly, that’s an acceptable motivation. In fact, I’ll go one step further. The Bible’s teaching is that God EXPECTS us to call him on his offer. That’s part of what faith is about--expecting God to keep his promises. From a Biblical perspective, to tell God, "No thanks. I don’t need or expect anything from you" is false humility and insulting to God.
Somewhere, there is a line that is crossed when we are just using others for our benefit. That’s the real issue, isn’t it?