Yeah, there’s the purely mercenary reasons, like better employment and salary prospects for U.S. workers who can speak Spanish, but Cindy Casares (Guanabee) came up with the best reason yet to learn the language of Cervantes…
The Mexfiles has never expected to turn a profit, but personal funds only go so far. Miscellaneous expenses (telephone, internet connection, subscriptions, spyware, hardware repair, etc.) run about 600 pesos (35 US$) a month, and donations are gratefully accepted. Please note that your donations are in Mexican Pesos (18 pesos to the US Dollar as of February 2019). A 25 US$ donation is 450 pesos.
The background of Mexican anti-clericalism and the "atheist" general who led the Catholic counter-revolution of the 1920s 128 pp., Editorial Mazatlán, 2012.
An oral history of the World War II experiences of Gilberto Bosques (1894 – 1997), Mexico 's Consul General in Marseilles, France, who saved tens of thousands from the Nazis. 36 pp. Editorial Mazatlán, 2007 $35 MXP (click the image)
mexfiles.net 
The best reason I know is post-facto: To understand these guys http://www.lesluthiers.org/
Here is a beter reason to learn spanish, you just can’t do this in english.
But first I should learn english “beter”
All the Spanish you really need:
http://d.yimg.com/static.video.yahoo.com/yep/YV_YEP.swf?ver=2.2.46One Semester of Spanish – Love Song @ Yahoo! Video
If I were to encourage someone to study Spanish, I don’t think I’d use the “Latin Lover” phrase. There’s an old (and in bad taste) joke about a guy who thought he was a latin lover but he was just a ………. Mexican. Just don’t go there. Instead, I’d say something about there being no place in the world where a second language was a disadvantage, especially here among the border states. I’d also mention that, if English is the person’s first language, learning Spanish, in annotative translation it is very close to the King’s English, would improve one’s English. An’, Sweet Thing, ain’t nuttin’ wrong ’bout dat!
English is probably the most unromantic language there is