Because most guys are not always sincere. Most of the time, before a girls finds a girl who's actually being sincere she finds guy who are just complete immature a$$holes and don't know how to respect a woman. So they feel the need to over analyze every little thing.
When a dog doesn't behave, some people resort to choke chains.
If the dog keeps ignoring its instincts and just follows through with its urges, it will get choked.
The dog will eventually learn to hold back.
I guess it's because (I'm sure) almost every girl has believed a guy's words only to find out their intentions weren't sincere.
Fool me once, shame on you...Fool me twice, shame on me.