Most Helpful Guy
There are definitely guys who will act nice because they think that's their ticket to your vagina. Some of them are slick pick-up artists and these are the "nice guys" women usually fall for that end up burning them. Even some of the more introverted "nice guys" are just after sex though; because they may be lacking in other areas of attractiveness, they see "nice" as their best option.
So yeah, a dose of skepticism is healthy, but if you're being paranoid to the point that you're dropping any guy who is nice to you from consideration because he *might* have a selfish motive, you're doing yourself a disservice.