Why are a lot of girls, (I understand not all

) so bloody insecure?
I know a lot of girls who are
really insecure, my cousin's ex for example. She's 17 he's 16, she's having his kid (early I know), he has a very good paying job, and I
know he loved her to bits, and wouldn't dream of cheating on her. Yet for some reason, she accuses him of shagging nearly
every girl he's in contact with. It pissed him off so bad that they split up, and I don't think he wants her back (though he says he is definitely
not going to neglect his daughter when she's born). Now the poor kid's gonna have split parents before she's even born, because my cousin's ex was so insecure.
Another example.
My bloody MSN pic! It's of me and a girl when I was out with my
mates. And I mean
mate. The girl in the pic with me is one of the best friends I've ever had, and she means the world to me, and I love her as a friend. Yet I've been told by my other mate's girlfriend that if
my girlfriend sees the picture, it would make her feel jealous and insecure! WTF??!!
I love my girlfriend, but I'm not giving up my friends for anyone or anything, and I'm not gonna just stop having pics taken with
mates just because they're female.
This is something that's been really pissing me off. I understand there are girls that aren't like this, but from what I can see, most girls are.
Anybody like to tell me
why? 