Lobbying in its purest form is supposed to be a way for people to have a voice in their government. Anyone in this country can ring up, or write, or email, their representative and push for an issue they feel strongly about. Likewise they can also make campaign contributions to the representatives, or candidates, they like and want to see in office. And the two can very easily be tied together: "I'm donating to your campaign because ...". If you keep it that simple it's a pretty good idea and is one that I think most people can get behind.
However in our modern world it really is just blatant bribery. Because corporations and rich individuals are able to donate such vast amounts that they are the only ones people care about. Take a look at how much money is spent during election cycles and it's obscene how many millions are poured into running for office. All that money has to come from somewhere. Say, for arguments sake, you must raise at least $5m to have a chance of winning an election but you think you can only raise about $2m from small donations (individuals) and then a few companies come along offering you nice big money bags if you agree to support their causes. Who do you care most about?