Finns feel bitterness towards the Swedish, because the Swedes don't have to live next to Russia*
But seriously, the thing is, even though only about 5% of Finns speak Swedish as their mother tongue, EVERYONE has to learn it at school...to some degree at least.
I don't really understand why most Finns whine about it though, Swedish is one of the easiest languages in the world - in terms of grammar at least. And if you speak some other Germanic language, you won't have problems with pronunciation either. But I guess there's the problem, Finnish isn't a Germanic language and Finns have a hard time learning a language they'll hardly ever need to use
*Some trivia: Finland has the biggest artillery in Western Europe - I wonder why