Why are people so stupid?

one of my best friends is making a hugh mistake with marring a guy she dated over myspace and cheated on her and called off their engagement once already and they have only ever seen each other in person once for four days ad all they did was have and the second day of ever seeing and knowing her in person he ask her to marrie him and being the way she is she said yes. i just want to know why some people are so stupid and go back to be like that