Ni3hao3!  One of my interests is languages: their history, how they're structured, grammar, syntax, writing systems, etc. I'm fluent only in English, but I've studied both French and Mandarin Chinese. It was probably the studying of French that interested me; English borrowed perhaps the majority of its present-day vocabulary from French, and this becomes obvious pretty quickly.  I would estimate that in each French vocabulary lesson I studied, about half the words resembled English words with the same or similar meanings.  For me, that revealed an underlying structure and history that still fascinates me.

As for Mandarin, I studied it for the intellectual challenge, and because I was curious about the Chinese writing system. About halfway through Chinese I, Chinese characters stopped being foreign and intimidating, and became, if not familiar, at least something that seemed more human than alien (a perception has stuck with me, even though I haven't studied the language in years). 

If you're interested in languages, I would strongly recommend The Cambridge Encyclopedia of Language and The Cambridge Encyclopedia of the English Language, both written by David Crystal.