Love and Fun aren't synonymous though they are similar, and sometimes love can be used in place of fun (usually not the other way around, tho). Fun typically refers to the pleasure you get from... well pleasurable activities; love is usually a state of being, for instance, I always love my dog, but that doesn't mean I always have fun with my dog (like when I have to clean up it's poop, for instance.)
Love is a feeling in and of itself, "I love this, I love that", the feeling/emotion of affection. whereas fun is the feeling derived from something "I have fun DOING this, I have fun DOING that" and in contrast, is not an emotion.
Hope this helps!