Why is "Settling" a Dirty Word in Romance?

While reading message boards online, I have happened upon a phrase that stuck with me. The reader said that she couldn’t stand to see how “Heroine X settles for Hero Y”. She expected her heroine to find love with another, and regarded her turning to the hero and her finding love with him as ‘settling’. I have come across this phrase more than once in comments various places, usually when a romance heroine has the choice between a very compelling love interest and one who is slighly less fascinating, often a beta hero. If you think about it, how often does the heroine choose the less compelling, but more stable, love interest? Very rarely! Why is the more rational choice of a life partner called ‘settling’ and frowned upon in romance?