Perhaps I’m too simplistic in my understanding—I always thought it related to the literal sealing of an envelope. In addition to the requisite saliva to activate the glue, the bonus is a little kissy-poo. But maybe there’s a deeper meaning of which I’m not aware.

I’m with @picante – I thought it related specifically to love-letters.

I think it’s a metaphor meaning , “I’m bringing this call or note or email or visit” to a close with a kiss. Meaning, they’re signing off with affection.

In the days when snail mail prevailed, the sender signified their romantic love with this short sentence.

In this day of e-mails, text messages and tweets, the usage may have drifted from those days of primitive written communications.