The number one top reason I do it is to make sure my zipper is up so I know I'm not exposing myself lol. I always check before I walk out of the bathroom but then forget I checked so I check once after I walk out and then usually again a few minutes later to make sure the zipper stayed up. I guess I'm OCD about that and it probably looks like I'm grabbing myself all the time but really I'm just making sure my zipper stays up lol.
Number two reason may be that somehow my penis is not sitting right in there lol. It's supposed to hang down most of the time but sometimes the way you move can pull it to the side in certain clothes and make it feel uncomfortable and it's so uncomfortable you can't stand it because it's sensitive so I need to move it ASAP no matter who sees me but I try to do it fast and not obviously.
Number three reason is maybe I'm dumb that day and am not wearing the right sized underwear or pants but am wearing them just because I think I look better in them but then I end up looking like a dork because I'm adjusting myself constantly lol. I think guys that grab themselves a lot are like that, they're wearing clothes that aren't comfortable because all they care about is if it looks cool or not.
I guess it's the same as why a woman would adjust her bra...yep that's right to get a little bit more comfortable ;)
Shame on you fellow men, giving away our secrets...
I only do it when my d*** or balls are in a painful position, so I can readjust them.
Adjusting theirselves :P