Some people (even poly people) really don't like explicit declarations of love, for a variety of reasons. Admittedly it's a word that come with a lot of social baggage and expectations -- because declaring love is a typical major benchmark of progress along society's standard relationship escalator
But if you're not hear the L word and that bothers you (or if you're just wondering why), it's probably a good idea to talk to your partner (and metamours, if any) about it.
Sometimes existing primary-style relationships have an agreement (or tacit understanding) that you "can't fall in love" with a non-primary partner. This is usually a way of establishing hierarchy, and an attempt to try to prevent jealousy or to keep a non-primary relationship from becoming "too important." Whether or not this is a good idea or if it works, it happens.
Trouble is, often existing primary partners do not clearly articulate this boundary or rule up front to non-primary partners. Perhaps they haven't even discussed it clearly between themselves, or perhaps they're just embarrassed to admit it -- especially after a non-primary relationship has gone on for awhile and the people involved are obviously pretty invested in it. A lot of drama can ensue in such situations -- which a clear, direct conversation could often prevent.