Answer:
The real answer is that Russia needed a port in the black sea for trade and influence. Also the old Russian empire fought the Ottomans to gain this territory. Back then in the Russian Empire era, There was no such thing as Ukrainian country nor a group. Even the now days Ukrainians have a different culture and dialect. West, East,North and east Pontic steppe which are way more closer to Russia. Russia believed the Crimea and eastern Ukraine belongs to Russia. The reason for today's hatred of the Ukrainian people towards the Russians is that during Stalins soviet era, he starved many Ukrainians over 5 million died. The treatment of the Ukrainians was so bad that even when the Nazis arrived in the Ukraine during WW2, it was seen as liberation.
Step-by-step explanation:
Main reasons
1. Influence and trade
2.Crimea is majority Russian.
3.Ukraine was created in 1918 and was never really a country. The region of the Ukraine was inhabited by many peoples over history like golden hordes, the Turks, Russians, Germans, Romans. So Russians view Ukraine's legitimacy very low.
4.Eastern Ukrainians want to be a part of Russia and western Ukrainians want to be left alone.
5.Putin wants to revive Russia to be a super power again.