Both were tagged as Harem and romance.
Both lead male characters got into the harem by accident.
Both harems often give their male characters a headache or other bodily sufferings that they received throughout the season of the anime they belong to. .
Both lead male characters are not doing very well with their studies (not making it to "Tokyo University" also qualifies).
Both lead male characters has one female member of his harem whom they feared the most.
In one anime it is the harem who came to the lead male character, while in the other anime it is the lead male character who came to the harem.......by accident.
One male character (Keitaro Urashima) is a hopeless romantic (not until Naru declared her love for him) , an accidental (and unlucky) perv and a wuss, while the other one, Chitose Hitotose is a bad ass and a laid back loner.
Though both animes have a "Harem" tag, in "Love Hina" Keitaro is a (reluctant) lolicon* co'z he's older than the women in his harem (according to info there is a 2 year age difference between Naru and Kietaro), while in "Happy Lesson," the harem was mostly a bunch of sotacon* ladies who have the heart (or hots) to help Chitose.
*it maybe a bit off but, I just like using the terms.
In "Love Hina" Keitaro Urashima ended up with Naru Narusegawa (which is obvious), while in "Happy Lesson" though Chitose may have kissed Mutsuki Ichimomji ,the story it still left unanswered as to who Chitose really likews among the 5 women who were in wedding gowns (final OVA).