Most Helpful Guy
From my experience it's because they believe they have to fit women's (often high) standards and out of concern of a better man "stealing" their woman's heart away. So if they like a girl then men often work harder to impress their significant other and expect less in return.
Sometimes it is fake sometimes it isn't, depends on the guy.
Most Helpful Girl
It is fake. Players especially do it to earn your trust. That's what they're called players for.