Just to throw some TLK fanart out there...
Taka ends up really hating Laguna when they're growing up together simply because Nakato starts paying more attention to Laguna than she did to Taka, and since he already has some parent issues, he gets jealous.
So he kinda taunts Laguna, about being born without knowing who his father was, and what not.
Scar (C) Disney
Laguna (C) Me