Translation:I love it when you smile, especially if the reason is me.
Jack, not only is there nothing wrong with this phrase (as long as it is used correctly in the sentence), but also your phrase is needed -- as I presently explain. Something like this is necessary to fix the bad grammar of DL's translation: "I love it when you smile, especially if the reason is me". No good! An object pronoun can never take the role of "predicate nominative". But since "the reason is I" would not be colloquial English, the translation can correctly be replaced by "I love it when you smile, especially if I am the reason".
Two possibilities I think: 1)You left out the "it" in "I love it when you smile...". In British English at least the majority of people would say it this way, I'm not sure about American English. 2) Again in British (and I am pretty sure in American English too), one would say "...especially if the reason is me". (See also the link in my reply to GeorgiaRom below).