There are lots of reasons. Let me start with the conjunctural ones.

The ruling party (PSOE) has been in a weak position in the last few years, with a wobbly parliamentary coalition and issues with corruption. The Spanish population generally likes international positions that could be summed up as "let's chill out with everyone and oppose war" (I'll comment on the nuances later), and the PSOE has found itself in a position where representing those ideas gives them an immense advantage in the polls for relatively few cons. In comparison, the two largest opposition parties (PP and Vox) have generally been siding with Trump and Israel, and the PP's leader ONLY made a milquetoast comment against Trump after he threatened to "end the Iranian civilization". This should make it clear that, if the last elections had gone in a different direction, Spanish foreign policy would have been quite unremarkable for current EU's standards.

But why is taking these positions so profitable for the PSOE? For starters, Spanish society is still marked by the 40 years of Francoist dictatorship, so any picture that has to do with the army has different connotations in our culture than, say, France. The army has to do its best to show its "we're here to save people in case there's a climate catastrophe and look pretty the rest of the year" face while the "we're the state's instrument to exert international power projection" face has to be shown with modesty, because otherwise it gets millions of people to remember ugly family stories from 50 years ago. This has the positive side that we're quick to react negatively when someone starts a war, but the negative that we're also reticent to involve ourselves even in favor of the good side (note how, even though supporting Ukraine is a very popular position, sending them a lot of weapons becomes uncomfortable, as you can see by the fact that Spanish support of Ukraine is modest in comparison of that of other European countries).

Then you have the fact that the US as a country has never been uncontroversially popular in any political tradition here. When we were first pressured to join NATO 40 years ago, PSOE's slogan was "OTAN, de entrada no" (roughly "NATO, no way we're joining"), because the left-leaning side of Spanish society slightly leaned towards the Soviet interpretation of the Cold War. And yet, the PSOE itself decided to change its tune later on, in order to avoid issues with the US and promote integration within the EU. So, what position did Alianza Popular (AP, today's PP, the right-wing heirs of the dictatorship) take on that issue...? Its leader told his followers to vote whatever they felt like. Spanish economic elites wanted to join the EU, and wanted to get along with the USA, but among the right-leaning portion of Spanish society, which leant towards the Francoist interpretation of geopolitics (and not the Western one), joining NATO was seeing as giving up sovereignity to the yankees. Regardless, the country was told that joining NATO was important, that we had to be a modern, democratic, European country, and it was accepted, but through a lot of controversy. And this still plays a role today. Even though right-wing media and parties are all clearly in favor of the US no matter what they do, right-wing voters like seeing Sánchez stand up to Trump for their own historical-nationalist reasons, while for left-wing voters it's a very obvious win from the get-go.

Then we jump to 2001-2003. The PP had taken the government from the PSOE after the latter had gone through huge cases of corruption (you should notice that this is common in modern Spanish history, both for PSOE and PP, and other smaller parties too), and they had become incredibly popular thanks to the development bubble they had promoted (it wouldn't crash until 2007). But then, the PP decided, for no benefit to Spain of any kind, that we had to join Bush in his invasion of Irak. Spain didn't really actually do much in the invasion of Irak, and we mostly took a support role for the US, but it was still incredibly controversial, triggering some of the largest demonstrations in modern Spanish history. When we got closer to the next elections, the PP had taken an important hit, but they were still projected to win with a safe margin. Then we suffered the 11M terrorist attack, three days before the elections. And rather than getting the US reaction of "we gotta get back at those damn Muslims", a lot of people seemed to have thought "yeah, no wonder, we did get into a war, after all", and the PP lost when no one expected it. From that point onwards, it's been a given that Spanish society favors non-interventionist by a wide margin.

Then you have other factors that aren't so sociologically or historically important, but still play a role. Israel doesn't lobby Spain so hard as it does UK and Germany. Spain has considerably less exports/imports with the US than other EU countries. We're a fairly important trading partner with Algeria, and international relations are fairly important when Algeria negotiates trade. These aspects may be somewhat minor when you look at the numbers, but they still push the tips of the balance in the same direction.