If Hikaru and Kaoru both wanted something, and that something was one-of-a-kind, they either shared it, or one had to give it up. More often than not, Kaoru would be the one to do this, not because Hikaru was pushier or greedier - neither of which were true - but because Kaoru saw no reason to take something his brother wanted. The younger Hitachiin twin's feelings were that, if Hikaru was happy, there was no need to selfishly take a material object. Hikaru often saw through Kaoru's weak excuses, however, and many a time, the older brother made sure his younger brother was the one to get what he wanted. Unlike most siblings, the Hitachiins argued more along the lines of "You take it!" "No, you!" when both of them wanted the item in question. Each valued each other more than anything or anyone else.
Kaoru had made bigger sacrifices, of course; as soon as he'd realized his brother's growing feelings for their friend, Haruhi Fujioka, Kaoru had abandoned his own feelings. Hikaru would never have to know. Besides, Kaoru reasoned, Hikaru's love was much stronger than his own, so it would really be selfish if he pursued Haruhi himself. When other friends among the Host Club realized the sacrifice Kaoru had made, they were stunned. Even the unshakable Kyouya Ohtori had stopped scribbling in his notebook for a moment to look up in surprise at the calmer twin's resolve. After then, Kaoru was always deemed as "selfless" and "giving" by those who knew him best.
Though the truth, of course, Kaoru would only admit to himself, was that he wasn't selfless at all, but just the opposite. For though he'd harbored a quick and painless crush on Haruhi, there was another for whose attention he'd longed for quite some time. Though he and his twin mocked the Host Club's president often, Kaoru had been quite taken with Tamaki Suoh for his generosity and his genuine desire to make other people happy. Tamaki, Kaoru figured, was truly the selfless one. He had been the one to bring these unlikely candidates to the floor as the Host Club, and they had bonded only because of his efforts. Something about the twinkle in the blond's violet eyes had caught Kaoru off-guard.
Tamaki's feelings were clearly directed elsewhere, though, at one ever-oblivious Haruhi. Kaoru had known it was a lot to hope for, but, still, the sensation of unrequited love was painful. To see Tamaki pine after Haruhi day after day was unbearable, which only drove Kaoru further.
As soon as he'd seen that Hikaru loved Haruhi, Kaoru had pushed for his brother to pursue his feelings. Many saw it as selfless and kind, but it wasn't, and Kaoru knew it. Yes, he wanted his brother to be happy, and he genuinely did want Hikaru to find said happiness in Haruhi, but...if Haruhi fell in love with Hikaru, Tamaki would graciously back down. He would put his friendship first. He would want Haruhi to be happy. He would do this because Tamaki was a selfless person.
He would get over her in time, and when he did, Kaoru would finally show people how selfish he could really be.