On the 28 February 2022, four days after the Russian invasion had started, Ukraine applied for membership in EU. What were the long and short term causes behind, and the trigger of, Ukraine's resolve to apply for membership in the EU?
The short-terms one(s) are a bit obvious as (according to polls) the desire of the surveyed Ukrainians to join the EU jumped up dramatically since broad-front invasion by the Russian army has started this year. Basically, once Russia has declared something close to total on Ukraine, the bridges are burned.
As for the longer terms ones, those are bit more debatable, but if we go by the geographical distribution of the Ukrainian population that does desire to join the EU, which is mostly concentrated in Western Ukraine, Russia taking away in various ways the Russian/Russophile population in Eastern Ukraine in various ways, be it by annexing Crimea or by supporting the self-declared republics of the Donbas, essentially had the same effect of diminishing the pro-Russia population that still lives in areas controlled by Ukraine's government. So in some sense, Russia has been pushing towards the EU the areas of Ukraine they haven't annexed or subverted for quite some time now.
Ukraine itself doesn't have that much of oil/gas-driven economy, so integration with Russia instead is of somewhat questionable benefits. It seems Ukraine mostly exports to Russia aluminium (oxide) and iron products, which I suspect are enabled by imports of cheap energy from Russia. (I don't know how much of that production happens in areas that Russia has already conquered in 2022, but I suspect it's a non-negligible proportion given Azovstal etc.) So while breaking ties with Russia was likely to disrupt those Ukrainian industries, the destruction and occupation brought by the 2022 large-scale war, ironically probably made that easier in the long run. Furthermore, if one takes into account the near complete lack of Russian investments in the reconstruction of a client state like Syria, territories that are aligned with Russia but not 100% physically controlled by it don't seem to fare too well. (Russia did rebuild Chechya, in contrast.)
As for concrete benefits of joining the EU, those are essentially the same that have caused most of Eastern Europe to want to join the EU. Their population eventually gets access to the higher-paying jobs from further West (and in Ukraine's case that probably even includes Poland). That eventually puts pressure of salary growth "back home". It looks like after 2014 more Ukrainians found work in the EU than in Russia, although that might involve some obscuring of the fact that some Ukrainians were incorporated into Russia that year.
The EU also funds development of the poorer areas with their structural funds; the alignment of various standards makes it easier for EU business to expand in the new territories, which generally results in an increased standard of living for most inhabitants, even if there are issue with some industries being pressured out, e.g. coal in Poland... of which Ukraine also has some, but again, I think mainly in the Donbas, so ironically again the Russian (or proxy republics) occupation in that are make the problem "go away" for the rest of Ukraine/EU. Given that Ukraine is a large agricultural producer, which is probably not going to change unless Russia occupies much more of Ukraine than it does now, I suspect EU-membership negotiations on that chapter are not going to be easy though.
Generally speaking however, in the long run, Eastern European countries that have joined the EU have fared better than e.g. Belarus in terms of GDP growth/capita (even at PPP), which has more closely stayed in Russia's orbit. (Somewhat interestingly, Serbia didn't fare that much differently from its neighbors, despite its political flirtations with Russia, but economically Serbia isn't that integrated with Russia, relative to the EU.)
There are many economical and other similar benefits from being part of the European Union. These are covered well enough in other answers.
While Russia did made economical progress, it is not seen by many as equally great and attractive. It may still be associated with the Soviet Union that was not particularly successful. The belief about the wonderful Europe where the hard work is converted into money and wealth so easily has roots in Soviet times and remains alive maybe because it is not completely groundless, even if at times slightly exaggerated.
Russia could easily fix this problem by achieving good salaries (for the own citizens first), funding joint research programs, getting ahead in advanced technologies. But all this needs money, money, money and money, you must win with the economy first to use such things for boosting the reputation.
This answer sets out to identify the factors behind Ukraine’s desire for full membership in the EU.
The long-term cause behind desired full EU membership would be their sense of identity with Europe, due to geographical location. Everything from the Ural Mountains in the east unto the Atlantic Ocean in the west, and from the Arctic Ocean in the north to the Mediterranean Sea in the south is Europe. 100% of Ukraine lies within these perimeters, in contrast to Russia which is located across both Europe and Asia.
The short-term cause behind desired full EU membership seems to have been the growing economic and social cooperation between Ukraine and EU that gradually emerged after the dissolution of the Soviet Union in 1991. Integration with Europe being a declared big goal in Ukrainian politics already in 1993, according to the linked website.
The trigger for taking the final step and applying for full EU membership, however, obviously was the Russian invasion of Ukraine, which rapidly pushed Ukraine away from Russia into the arms of the EU.
Consequently, major geographical, economic, social, and security factors influenced Ukraine to apply for full EU membership.