I wonder if part of the reason that people have a hard time believing love can be strong and powerful outside one-on-one exclusive couples is that many people confuse the heady rush of initial infatuation with love.
So often, I've seen couples talk about trying to "restore" love to their relationships when it seems to me that their love is already deep and strong. What they're really missing, or at least it seems to me sometimes, is that chemical rush of initial "falling in love," which I'm sure we can all acknowledge is pretty special. Who wouldn't want to experience all those wonderful feelings again?
I think memories of falling in love are intense and beautiful for many of us. But the idealistic narrative that couples can experience those feelings for each other all the time or very often is bound to disappoint, isn't it?
So if we already tend to confuse infatuation with deep love, if we have a hard time thinking about them as separate things even to ourselves, wouldn't it be natural that we would tend to feel jealous about a partner having strong feelings of infatuation for somebody else?
I don't really know exactly where I'm going with this other than to suggest that understanding the different kinds of romantic love can help us multiply love rather than ration it.