Is there any reason why Diesel costs more? I was always under the impression that it was a less-refined fuel therefore cheaper.
While diesel is less refined, the difference isn't much. These days both are heavily refined products, and the cost to refine gasoline isn't very much per gallon.
The real answer: supply/demand is the key factor in the price you pay. Cost to create/build a product generally sets a floor on the price you pay: a business that isn't making money will stop making the product (loss leaders exist, but not a factor here).
That's the problem with hindsight theories, especially if only using a limit horizon into the past.
Over the past 20-ish years fracked oil, which is mostly too light to cost effectively refine into diesel but makes nice gasoline, has made up most of the increase in oil production. At the same time, conventional crude oil production has been slowly declining. Conventional crude is quite easy to refine into diesel. Heavy/sour crude replacements are more expensive to refine into diesel because more work is needed; also preexisting refineries need refitting to support it.
The net result is that diesel production hasn't kept up with the wider percentage increase in oil-based fuels and refinement is more expensive because it's a choice between cheaper oil which requires more work and costly refits, or less-available and therefore more expensive base oil. Thusly diesel is more expensive due to cost of production and reduced relative supply.
[0] https://www.npr.org/2022/08/31/1120422634/breaking-down-the-...
This started, I believe when fracking became a major thing (the late 2000s), because shale oils tends to have less heavy hydrocarbons then the old wells.
And the ethanol makes the fuel less valuable (energy reduction per litre) moreso than biodiesel.
Jet fuel and diesel fuel come from similar parts of the barrel. Refineries can largely choose what kind of fuel they produce for a given unit of input. One barrel doesn't always split into the same fractions. There is configuration involved.
I was always under the impression that a big part of the difference in cost is largely due to taxes.
(Was that enough of an ego stroking for you? I can praise you more, if you'd like.)
In the US diesel taxes are often different from gasoline, but polluting is generally not a factor.
EDIT: meaning "less taxes"
No. That's not correct. It got more expensive than gasoline since the attack on Iran.