BE is because I essentially want mine to get bigger, but I don't like surgery. Also, the speed at which it happens in Hentai and such is a turn on by itself, and the imagined weight. (I'm not worried about long-term problems because I can counteract the weight with exercise).
Lactation is hard to explain. A combination of liking the flavor, wanting to essentially induce nearly permanent lactation (for growth mostly, but it also has other perks.) And for some reason it just amuses (read arouses) me more than it has any right to, and somehow seems strangely intimate at the same time. (I guess sustenance from your partner?)