If I'm in McDonald's and a fat person enters that does not increase my chances of gaining weight.

If I'm driving and a driver not wearing a seatbelt enters the road it does increase my chances of being in an accident because if something happens that makes them have to swerve sharply, such as a large animal leaping into the road in front of them, a seatbelt increases the chances they will be able to remain in control during that maneuver.