Russia and Ukraine are similar in culture and language and I know they used to be the same country in the past. I am in Ukraine now and most of the people here speak Russian and Ukrainian fluently. Even the place where Russia started (Kiev) is here. What happened?
When the USSR fell, Ukraine (along with many of the Soviet states) decided that they wanted independence from this government that they blamed for so many issues (and that had just collapsed, not exactly reassuring people that they could provide good governance).
300,000 Ukranians formed a human chain demanding independence, and when they held a referendum on it, every region voted in favour (with Crimea being the lowest at 56% in favour of independence).
Ukraine had been one of the founding members of the USSR (it was it's own country/region before that, it didn't always consider itself to be Russia), so when the USSR collapsed, Ukraine decided that it was better to be independent again, and not try and stay in a failed union.
Before the USSR, Ukraine had a long history of having its own national identity, even when occupied by other countries, so whilst it has strong ties to Russia, it was it's own state first and foremost, with the Russian part always being secondary.
Russia did not start in Kiev. Kiev was an important city of medieval Rus' (East Slavic lands), hence "Kievan Rus'". Russia originates from the Grand Duchy of Moscow.
Russia and Ukraine used to be the same country because Russia got most of East Slavic lands of Poland during the partitions in 1772, 1792 and 1795.