What's the problem with "personal" gain?

When we talk about a person acting for reason of their "personal" gain, it basically always implies that they are doing something bad and wrong. This is fucked up. There's nothing wrong with acting for one's personal gain. Just ask Ayn Rand!

But there often is something about what they're doing that we don't like or don't want. What is it?

Usually what we're getting at is that we have given this person power with the intent that they use it to benefit others — and they use it only, or inappropriately, for their own benefit. It's about how they are using the power others gave them, relative to what the others intended for them to do with that power.

Why is that a problem?

Let's say you are a colonial governor and you use your power to benefit the populace, rather than the empire to which they're beholden. From a modern progressive point of view, sounds like you're making the best of a bad situation and you should be commended. (You know, except to the extent you're obstructing la revolución. Because you won't be governor forever...) But you may not be using your power as the empire's officials intend or prefer. Does that make it wrong? Clearly not, from this perspective.

So, what's the problem with the gain being "personal"? I think the other assumption is that whatever power was granted to this person, we agree with the purpose it was intended for. The problem is that the person acting for "personal" gain is hindering or scorning a purpose we like.

But again, I'm left with the question: Why do we highlight the "personal" in personal gain? Is there really anything especially vile about its being "personal"? Is it only saying that we are uncomfortable with having individual, "personal" interests and desires that may differ from the group's desires? What's the problem with "personal" gain?