Liberals think that other people are selfish and untrustworthy and therefore should be forced to pay taxes to support the "good" things liberals want done. They also tend to think of themselves as both "good" and as having done their part by simply being liberal. Therefore it is just "fair" that they have special deals to let them avoid taxes.
Fundamentally, liberals and more broadly "progressives" demand big gov't and loath freedom because they fear others will use their freedom in ways that do not conform to the liberal notions of "good" and "fair".
Small "L" libertarianism which includes a broad swath of Main Street "conservatives" in America promote small gov't with less power because they respect the freedom of other people and loath gov't interference.
We ALL tend to think that others think or feel like us. If you are instinctively selfish (not saying you behave that way) then you are more likely to think others must be "forced" to share. If your instinct is that others ARE your responsibility and that you should help them directly then you tend to believe gov't is a huge problem when it comes to helping people.