I agree, in a vacuum, there is nothing wrong with the concept of lobbying. I would go out on a limb (well not quite a limb, I have family members that were lobbyists) and say the majority of lobbying that occurs in the US is actually good.
But where it's bad, it's really bad. The loudest and most influential voices are the ones with the most money, and sometimes that's good, but a lot of times thats bad. They promise golden parachutes and shower politicians with money so much so that even if the politician gets burned, they are promised a career at the lobbying firm or with one of the "hookups". Again this is bad, because now instead of being accountable to the constituency, our politicians are accountable to their benefactors.
There's a lot more reasons this is wrong, but this is the primary gist. I hope we can all agree this is an issue.