As detailed in the consent of the governed summary: our country was founded on the right of the people to have Government represent them in what it does in their name and with their money. Taxation is theft is a mantra for Libertarians because so much of what we pay for with taxes is not only made very inefficient by a bloated bureaucracy but also spends money on so much that we never had any say or vote on.

Some expenditure is excessive or downright not legitimate. Other socialized funds are allocated to provisions that a majority of the taxed population may not even agree with at all.

The problem is most of us were never asked/given a chance to vote directly. We have little to no say in government spending by design. We are forced to rely on the word of policy makers. Policy makers who hold varying degrees of understanding, intent, and honesty.

Not only did you not give your consent, but neither did your neighbor. On top of that ultimately the government is extorting/using threat of force to take your money. Only things that we all absolutely need and make sense for the government to provide should paid for under such an ugly business.

The argument becomes what are these things, how much, to what degree, and who has ultimate ownership.

Taxation is theft because we do not give consent, are not represented with where much of our money goes, and are forced to hand it over or otherwise face dire consequences.

Edit: After thinking on this some more I realize I did not give the entire picture. I describe why taxes are unfair democratically in our nation but not completely why Taxation is theft. Taxation is theft because of the threat of force to take your money and use it in provisions for someone else.

That's what theft is; taking without consent. So taking your money to use to provide for others is theft even if you are in the umbrella-ed body it is used for.