I am unclear why you think love would exist without sin? If no one sinned, then would "love" (defined as the willingness to forgive someone without punishment for their deeds) exist. Because love means so many things in English, it is important to narrow the definition when referring to what Jesus is talking about.

I found similar sentiment among all the world's religions when I studied them at a liberal college as a religious studies major. Christians assign God as the source of the strength to forgive, but other religions are not that different.

Stepping out of Christianity and simply looking at how societies operate. Seeking individual justice for wrongs against you is pretty destructive for a host of reasons. So, every society establishes a justice system and relies on it to administer justice, while individuals are asked to move on with our lives. Forgiveness facilitates that moving on process, but it is reinforced at some level knowing that a just God or hopefully a "just" legal system is looking out for the people as a whole.

Some religious studies experts believe we project onto God what works on earth. Others believe we mimic how "heaven" works using the Bible/Koran/and other religious texts to guide us. But no matter the source, forgiveness (love) is linked to sin (evil). Without sin, what is there to forgive?