Here's why:
The Stowaway

There's almost always this one guy that's so much in love with the heroine that apparently he has abandoned the idea of self-respect. I'm always a little sad for this character. This is sometimes the close friend that has been there through all the heroine's trials and has stuck by her through the worst of situations. If handled well, the stowaway can actually be good for the story, but sometimes things go way too far.
The heroine, whether she knows it or not, tortures him. He is more like a servant than a friend. He is at her every beck and call. It gets worse. Sometimes this character tries to move on, he tries to forget her. He knows she prefers the other guy and tries to back off...What does the heroine do? She gets jealous!

Whoa! I'm sorry that the stowaway decided to dig up some dignity and get over you. How dare he not remain as your back up?! Seriously?! Ugh! Get over yourself!
Little Miss Perfect

May I introduce you to Her Majesty, the Queen? *rolls eyes* She is perfect in every way. She is Belle, Ariel, Sleeping Beauty, Snow White, and Princess Jasmine all rolled into one. This type of heroine creates love triangles that aren't even love triangles anymore. They become some type of messed up love polygon with uneven sides.
This is just crazy! The heroine is so perfect that insta-love is bound to happen. People just fall in love with her off the street. NOBODY dislikes her, and if they do, it's because they're jealous. They're jealous that she just radiates awesomeness and they can't be like her.
And the love interests? They come out of nowhere. The stranger off the street is in love with her, her closest guy friends are fighting over her, the pizza boy is in love with her, guys suddenly forget they have girlfriends around her. All hail the queen!
Oh, come on! Sorry, but I like my heroines to be have little quirks and faults. I don't buy the whole "everyone is in love with her" business. Oh, please.