Why did Paul vote for tax decreases? Because the is a Libertarian.
He believes in smaller government, and fewer taxes. He also believes in a balanced budget and reducing the deficit and debt.
While it may seem like common sense that lowering taxes will increase the deficit, that is not necessarily the case. As a previous answer pointed out, decreasing taxes can actually increase revenue by stimulating the economy and increasing overall taxable output.
But, this is not Paul's only objection. There is no proof, that increasing (or not decreasing) taxes would in any way serve to reduce the deficit. This is because, the US government has a track record of refusing to decrease spending, and in fact almost always increases spending. The debt ceiling has already been raised several times to accommodate our ballooning debt.
For that reason, Paul prefers to advocate for lower taxes and more responsible government, rather than increasing taxes which will almost undoubtedly be used for increased spending in the future, and more debt.
Paul had introduced a plan to reduce the debt. He called this plan the "penny plan". In it he proposed a $400billion reduction in spending for 2019 and in subsequent years, to cap spending increases at 1%.
With just these modest limits, the budget would be balanced by 2023 and we could be paying down $700billion of the debt per year by 2028. At that pace the debt would vanish by about 2055.
You would think that Paul could get some traction with his ideas with a Republican President and Senate, but unfortunately many Republicans these days are conservative in name only. Instead of the $400Billion reduction Paul wanted, Trump increased spending by about $300billion this year. Trump is running deficits above the level of the Obama years, which is really pretty pathetic for a so-called "conservative".