you just gotta get creative. well actually its more real then creative; this is what i came up with:

it's safer- no shady characters to deal with and no chance of getting jacked for your money etc

it's cheaper-- weed today is just rediculously expensive

it's the only way-- wouldn't you rather me have to cultivate and care for the fruit of my labor before smoking it then getting $20 from you to buy food with and really goin' spend it on weed?

there's alot more, im sure you can come up with some shit