This is called "Heavy petting" It is sexual exploration and is often a prelude to sexual intercourse.
They are usually masturbating (or keeping their hands warm.)
It makes them look cool. It is a fashion statement or craze. Some guys think sagging is cool. Similarly, putting their hands down their pants as they walk along is just another craze. I hope they wash their hands before eating !
First they don't always look. Second if they are looking they are probally checking the boy out.
Sagging pants is a personal choice that guys can make. Most girls, however, agree that this practice is odd to look at, and would prefer to look at a cute butt. Guys sag because they think girls think its hot. But to most girls its weird IF THEY HAVE A PENIS THEN THEY SHOULD SAG, IF THEY HAVE A VAGINA THEN THEY SHOULD NOT SAG
sort of.
down a guys pants
Yes. Because I'm pretty sure girls find it gross if the guys have sweaty hands. Unfortunately .. yes.
So they have something to hold onto!
to make it easier to put on theirs pants. guys just use it for convenient bathroom plans.
on the guys shoulders
Because they want to be the first one to get in your pants. Some guys are just jerks
Guys like dirty things. I guess they just like looking at the girls bodies. :) Weird, but the truth. :P