@jobhunter7
I know it used to be part of the Russian Empire. But do they believe the same about Poland or Finland which were too? And what other former parts of the USSR do they believe to be still basically Russia.
If we look at some statistics about Ukraine, we can arrive at an answer.
Between 43-46% of people in Ukraine speak Russian. Crucially, about the same proportion of people speak Ukrainian.
Both languages evolved from a common ancestor language - Old East Slavic. Both use the Cyrillic script and are the only Slavic languages that do.
Despite this, they are NOT mutually intelligible languages. This is because the two languages evolved differently for 600 years and had other influences. For example, Ukrainian has a lot of Polish influences.
The peoples of Ukraine and Russia have a shared history, including ancestry. Similarly, the Spanish and Portuguese have a shared history in many ways.
The main reason Russia and Ukraine don't share the same outlook is basically Nationalist politics.
Russian nationalism views Ukraine and Crimea as part of Russia. The Soviet flag seen on a Russian tank recently demonstrates a desire for reforming the Soviet Union.
Ukrainian nationalism went a different route and desired independence as well as closer ties to Europe.
Other European countries have similarities in outlook like above. Spain once had the same view of Portugal as Russia does of Ukraine, but hasn't for a long time.
More relevant to the British Isles, Irish Nationalists and Republicans view Northern Ireland as not being a real country and theirs - despite the 1999 referendum vote in the ROI. Northern Irish Unionists and Loyalists wished for closer ties to the British and to be separate from the ROI - Similar to the Ukrainian sentiment.
More importantly, Russia and Ukraine do not have a Good Friday Agreement of their own. That's where the UK, Northern Ireland and the Republic of Ireland are more fortunate.
These are the main reasons why some countries view others as part of their own whilst other countries do not.
It's Nationalist politics. Often dressed up as something else. In Putin's case, NATO expansion.