My brief answer for both: God
My expanded answer: Love has always existed because God has always existed. As John has explained in his first letter, “God is love” and “love comes from God.” God is love because God is triune and altogether righteous, holy, good, and truthful. Without the triune God of the Bible there is no love.
An insurmountable problem for Islam, Judaism, and cults like the Jehovah’s Witnesses is the absence of any answer for the origin of love. In these religions God is a single person. Love is not possible unless there are at least two persons to express and receive love. For strictly monotheistic religions, a nonloving entity supposedly creates beings with the capacity to give and receive love. The claim is that a lesser entity creates that which is greater—a clear violation of the principle of cause and effect.
A common rebuttal from strict monotheists is the hypothesis that their single-person God had the capacity to experience love and created humans so that he could begin to give and receive love. In this scenario God is unfulfilled until he creates. He is compelled to create in order to experience love. For the trinitarian God, creation is not a need. It is an option.
The problem for polytheistic religions is the lack of a single essence among the panoply of Gods. This lack of a single essence results in different character attributes, goals, plans, and purposes. The resultant lack of complete harmony inevitably diminishes and even destroys love. Contrastingly, Jesus told his disciples, “Anyone who has seen me has seen the Father” (John 14:9) and “I am in the Father and the Father is in me” (John 14:11).
We experience love because God is love and love comes from God. However, the love we currently experience is not yet at the level that God intends. Once God permanently eradicates evil at the Great White Throne, we who are followers of Jesus Christ will become one as the Father and the Son are one (John 17:11). Our love for one another will become greatly multiplied as we more fully and intimately observe the Father, Son, and Holy Spirit loving one another and as we more fully experience love from the Father, Son, and Holy Spirit and from one another.