When you're in love with someone, you want to vocalize those feelings for that person. Of course in a serious relationship, both parties involved should know that they love each other through their actions moreso than through words, but if you really care about someone, why not tell them that whenever you want to? And for most couples who are really in love, they want to say it. And really, if you feel that saying it only once is enough, there's gotta be some issues there about opening up and really committing to a relationship.