Metaphor and Metonymy

I was wondering about something that was mentioned today during lecture. Lacan believed that a comparison of sorts had to be made first before something could take on the other object's meanings (the actress/star example Professor Doane used). If this is then the case, that the metaphor is formed and then takes on other meanings, how do we get the metaphor in the first place? What compels a person/speaker/writer to make such a connection in the first place if the meanings are not already related? Is the metaphor then completely arbitrary, and if so, how can it be?

(On a slightly unrelated note, I find it interesting that the word metonymy means literally "change of name." Since the signifier and the signified don't touch in a metonymy--which actually confused me at first, because I thought of the two as being rather reliant on each other--it is perhaps logical that the two concepts remain separated by that bar; the change of name, such as White House meaning President, seems to hint at no overlap, but maybe just a slippage. It refers back to the shifting desire, which can't be satisfied exactly because there is no overlap.)