One of the reasons why I feel that poly is a better fit for me than monogamy is because I don't feel like I need to restrain my feelings when I start to like a friend. That doesn't mean I want a relationship with them or want to have sex, but that it doesn't have to be as clear-cut as relationship/friends. It could be a close friend who I love but I'm not in a relationship with.
For example, there is one guy who I have feelings for (who I see quite rarely), but to me it feels more like fondness and safety/comfort...I don't feel like I want to have sex with him because it would ruin the purity of the feeling, nor do I want a relationship with him because I know we are too different for it to work. However, I think my feelings for him are clearly different from regular friendship.