In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Abu Hurairah [radiyallaahu-anhu] narrated: The Prophet [sallal-laahu-alayhi-wasallam] said, ‘’Love your beloved one modestly, for it may be that he will become hated to you someday. And hate the one whom you hate modestly, for it may be that he will become your beloved one someday.” [ref 1]
Ibnul Atheer [rahimahullaah] stated in An-Nihaayah 5/284 that this means: You should love your beloved one with moderation and do not go beyond bounds. Do not go into extremes in your love and hate, for it may be that one day you’ll hate the one you used to love and love the one you used to hate. Therefore, neither exaggerate in your love and become regretful [in future], nor exaggerate in your hatred and thus become shy in the presence of the one you used to hate after you stop hating him.
Ref 1: Sunan Tirmidhee 1997. Declared saheeh by Imaam Albaani in his checking of Tirmidhee]