I speak Castillian - and spend time pretty much every day trying to improve it.  For me, learning the language was a big part of living in Spain.  If I wanted to live in Spain and speak English all day I would go to Torrevieja.  I can understand some Catalan but I've not got the time to learn both.  I get a real buzz from being able to converse fairly freely in a second language, and that really pushes me on to learn even more.  

It's not just the words either - things don't translate literally and it's always a revelation to me to realise the meaning of something new, often from how it is used in context.  Just the other day I was watching a film (in English) but with Spanish subtitles (a great way to learn, by the way). Anyway, the Spanish for a monkey is mono or mona, BUT in a different context it also means cute, so if you hear someone referring to a baby as 'que mona' or 'que mono' you know they are saying  he or she is cute and not that it looks like a hairy monkey....

Another one is salida - yes, the exit, but if someone says to you '¿estas salida?' they aren't asking you if you're on the way out, but if you are feeling randy!  Careful how you reply to that one.....

Nigel