1) The US helped plot to overthrow the Ukranian government in 2014.
2) Russia was not keen on NATO pushing for Ukraine to become a member, which would literally put the Russians major warm naval port under NATO hands.
3) Crimea has been a part of Russia for centuries, the only reason Crimea became a part of Ukraine after the breakup of the Soviet Union is because the SSR transferred Crimea from the Russian state to the Ukraine state in 1954 to celebrate the 300th Anniversary of the Ukraine as part of Russia.
4) The vast majority of the population of Crimea has been for hundreds of years of Russian ethnicity and language.
5) Crimea voted to rejoin Russia
The State of Politics are much deeper than you seem to admit.
Edit: Russia's support for Syria has more to do with the strategic location of Syria and the transportation of Russia's Natural Resources through that territory.