Ukraine has been dominated by Russia for much of its history, especially the last hundred years where Stalin tried to wipe out the ethic Ukrainian population through deliberate starvation. Ukraine had been part of the Soviet Union until it broke up. Russia had based nuclear weapons in Ukraine and has its only year round Naval base in Sevastopol (other than the small one in Syria)
The countries that border the Black Sea are Romania, Bulgaria, Ukraine, Russia, Georgia and most significantly Turkey, which controls the access.
Russia is only a short ferry ride from the Crimean Peninsula when you here mention of Russia taking control of the ferry. There is also a rail yard right by the ferry useful for getting military goods to Crimea in a hurry.
To get to Ukraine from Russia over land without using the ferry, you pass through an undefendable border checkpoint on flat land in the middle of farm country. Since Ukraine had been part of the Soviet Union, this wasn’t really a border until about 20 years ago.