Most Helpful Guy
The reason is either one of ego or the desire to cheat, or both. Some guys can't resist having their ego stroked and their sexual appeal validated and there can never be enough sources for them. It's not enough to know that one girl is into them, they have to know that multiple girls can be into them.
Some guys will leave it at that, perhaps flirting a little but no more. Other guys will want to take it further, because no amount of validation is ever enough.
If a guy has a secret profile on the likes of Facebook that you aren't supposed to know about, then the likelihood is that he doesn't want you to know about it because what he does there might seriously piss you off.
Most Helpful Girl
Because they're attention seekers and want to get their game on.