Probably because we've read so many shoujo romcoms that we become wary when a certain character type gets introduced. If the two main love interests seem to be getting along, bam, it's time to introduce a love rival to stir things up and make the series more "exciting"... It's just a basic tenet of the demographic. Can you even think of one shoujo series that doesn't have a love rival or, worse yet, a bona fide love triangle???