If Gai's sole drive and focus to get Kaori to fall in love with him is that she didn't do so right away like he was used to with other women, then that is the very picture of entitlement, of being used to getting what he wants and reacting badly when something shakes and challenges that. As to why I hate him...the best example is the opening scene of episode 17, where he grabs Kaori and drags her away from the others, shoves her into an elevator, and then invades her personal space, demanding to know whether he loves or hates her while she is visibly scared of him. He is constantly trying to coerce and guilt-trip her into getting into a relationship with him, and when he finally gets what he wants from her, he continues to flirt with other women and acts like proper table manners and talking to her parents are just impossible burdens for him to bear.
It also doesn't help matters much that he was hyped all to hell and back as being this ultra super badass by the fandom, when really his acts of badassery, on the occasion when he had them, were only moderate at best.
Also, Raita is the normal guy of the group, an average country-bumpkin; Gai hits way too many masculine power-fantasies to ever be an "ordinary guy".