Primarily work related reasons - I am in IT and Russia is quite big in software.
But plenty of secondary reasons - E.g
I am Swedish, Russia is more or less a neighbouring country, fascinating and massively big.
Sweden has identified that Russian is a very important language for Swedes to learn, yet students rarely choose it and there is also a serious shortage of teachers. Government is at its wits end trying to get citizens to learn Russian. Same situation in Finland. I thought I'd set an example for others, lol...
Political/Economic - the anti-Russian paranoia that sometimes grips Western Europe is ridiculous and counter-productive and not in the interest of regular people. The best way to stop it is for people to be able to communicate with each other and have good relationships. Russia is playing well below its league financially; things can only improve on this front and it won't be long until Russia has a lot more dealings with Europe than just selling gas. That's when those who made the effort will be able to reap some career related benefits.
Apparently Russian is the most commonly spoken mother tongue on the European continent and one of the most common second languages. Fascinating country of contrasts and interesting people. Not to forget ex USSR countries where it seems that most people still quite happily speak Russian. Is that enough reasons?
The way I see it, it makes more sense for most Europeans to learn Russian than Spanish or perhaps even French.
The negative thing is that it is a lifelong work. Even native speakers of other Slavic languages (Poles for instance) think Russian is hard and that they can't speak it well. So this is discouraging... But I'll see how far I can get in a year before I jump to any conclusions.
Oh I forgot to say that I had a brief brush with Russian at school but dropped it because I thought it was too hard. I've never quite recovered from this defeat and now I want to prove to myself that I CAN in fact learn Russian.