Well chances are the girl will not let a guy hold her boobs so I guess that knowing they can't do that they look for another place to hold, so they go for the waist because it's relatively close to the vagina but not so close that it would usually make the girl uncomfortable (Assuming the two are lovers)
Note: This is only a theory and kwa no means should it be taken as a legitimate answer.
Well I really don't know WHY ...but I think it's kinda sweet..u know ..actually I like it, ofcourse not from all the guys, cuz if any bila mpangilio guy come and hold my waist the most probable thing is that he'll get kicked LOL ...but if it's a guy friend au boyfriend I think it's ok...btw girls also like to hold guys from the waist xD
because (and this is just a thought) they like the feel of the curves. anyway, if a guy held my waist....well, i guess it would depend on who it was... if i liked the guy, i don't think i would care much.... if i didn't....he's probable gonna have some problems...